Knox, Indiana (CNN) Jadon McDonald studies the picture book and reads the words aloud: my, he, she, the. He pours on the charm, using a deep-throated caveman voice and flashing a huge smile after each word.

Within moments, he announces that he wants to walk. He spins from the coffee table, grabs his walker and scoots across the living room.

His twin brother, Anias, sits nearby in a specially designed wheelchair, playing with a soft keyboard he can touch with his feet. His movements are more restrictive, his mobility more limited, his speech more strained.

The boys, born joined at the head, once had to do everything together. Now 3 years old, they're learning to navigate the world apart.

The difference in their progress isn't lost on their parents, Nicole and Christian McDonald. It's part of their everyday reality, but one they view with optimism.

Jadon is "like personality with feet," Nicole said; Anias is the "blank canvas waiting to turn into a masterpiece."

"He is a child without limits," she said.

Christian says Anias is equally remarkable, simply operating at a different pace.

"In this day and age, we're kind of a quantum leap society -- you know, we want quick fixes," he said. "Anias can do amazing things and will do amazing things. But it's not just a quantum leap. It's something that's going to happen gradually."

Jadon and Anias captivated millions around the world when they underwent a 27-hour separation surgery in October 2016 at the Children's Hospital at Montefiore Medical Center in New York.

The twins, who were 13 months old at the time, shared a 5-by-7-centimeter section of brain tissue. Studies have shown that 80% of these so-called craniopagus twins die of medical complications by the age of 2 if they are not separated.

Photos: Conjoined twins separated: New life, apart Anias, left, and Jadon McDonald were born conjoined at the head, something only seen in 1 out of every 2.5 million live births. They were separated in a 27-hour surgery at the Children's Hospital at Montefiore Medical Center in New York in October. Hide Caption 1 of 24 Photos: Conjoined twins separated: New life, apart Nicole and Christian McDonald talk with Dr. Sanjay Gupta in the family waiting area on October 13 as a team worked to separate Jadon and Anias. "When we sent them off this morning, to me, I felt at peace with it and just ready to handle what comes after," Nicole said. Hide Caption 2 of 24 Photos: Conjoined twins separated: New life, apart Dr. James Goodrich Goodrich, left, leads a surgical team as they prepared to separate the twins. "Failure is not an option," Goodrich told the team as they got started. Hide Caption 3 of 24 Photos: Conjoined twins separated: New life, apart Hide Caption 4 of 24 Photos: Conjoined twins separated: New life, apart The twins' surgery was Goodrich's longest craniopagus surgery. It's meticulous, tricky and complex: A single cut too deep can lead to catastrophic bleeding. Hide Caption 5 of 24 Photos: Conjoined twins separated: New life, apart Goodrich's team worked more than 16 hours just to separate the boys, and each continued surgery individually afterward. Hide Caption 6 of 24 Photos: Conjoined twins separated: New life, apart Newly separated twins Anias, left, and Jadon in surgery at the hospital. Goodrich informed the family of the successful separation at about 3 a.m. October 14. "Well, we did it," he told them. When it was official, the room burst into spontaneous applause.

Hide Caption 7 of 24 Photos: Conjoined twins separated: New life, apart Jadon recovers in the pediatric intensive care unit shortly after the surgery. Hide Caption 8 of 24 Photos: Conjoined twins separated: New life, apart Anias returns to his room after his head dressing was changed. Hide Caption 9 of 24 Photos: Conjoined twins separated: New life, apart Jadon stretches his arms in his room within the hospital's pediatric intensive care unit. Anias rests in a nearby bed in the same room. Hide Caption 10 of 24 Photos: Conjoined twins separated: New life, apart Anias, left, stares at Jadon for the first time since the surgery that separated them. Hide Caption 11 of 24 Photos: Conjoined twins separated: New life, apart Nicole McDonald, right, and her mother, Chris Grosso, with Anias in mid-November. Anias had to have his skull cap removed due to infection, but doctors say they are still pleased with his recovery. Hide Caption 12 of 24 Photos: Conjoined twins separated: New life, apart Anias, left, and Jadon lie in a red wagon at the Children's Hospital at Montefiore Medical Center on December 13 as they prepare for the next stage of their journey, two months after their surgery. Hide Caption 13 of 24 Photos: Conjoined twins separated: New life, apart The family was headed to the hospital's banquet hall on December 13, where surgical and pediatric intensive care staff members were gathered for a farewell party. Hide Caption 14 of 24 Photos: Conjoined twins separated: New life, apart Dr. Oren Tepper, the twins' lead plastic surgeon, holds a thank you gift from the McDonald family presented to him at the farewell party. Hide Caption 15 of 24 Photos: Conjoined twins separated: New life, apart Nicole McDonald holds Anias as his twin brother, Jadon, sleeps in the bed to the left. The twins' older brother, Aza, watches television at the hospital from one of the boys' beds shortly before they left for rehab. Hide Caption 16 of 24 Photos: Conjoined twins separated: New life, apart Jadon, left, and Anias McDonald look up at hospital staff as they leave their room at Montefiore Children's Hospital in New York. Their older brother, Aza, proudly sits at the front of the wagon. It was mid-December and they were headed to rehab. Hide Caption 17 of 24 Photos: Conjoined twins separated: New life, apart Nicole McDonald pushes a stroller with Anias as her husband Christian pushes Jadon down a hallway on June 14 at Blythedale Children's Hospital in Valhalla, New York, where the boys have been rehabilitating. Hide Caption 18 of 24 Photos: Conjoined twins separated: New life, apart Anias plays with a toy while laying on a mat in his room at Blythedale Children's Hospital. Hide Caption 19 of 24 Photos: Conjoined twins separated: New life, apart Jadon eats small snacks and drinks from a sippy cup on his own, major progress since he first moved to rehab. Hide Caption 20 of 24 Photos: Conjoined twins separated: New life, apart Nicole McDonald plays with her son Anias as Christian McDonald holds Jadon on a playground outside Blythedale Children's Hospital. Hide Caption 21 of 24 Photos: Conjoined twins separated: New life, apart Nicole McDonald holds Jadon while looking over discharge information on September 1, as they prepare to leave the rehab facility and head home as a family for the first time. Hide Caption 22 of 24 Photos: Conjoined twins separated: New life, apart Nicole McDonald takes Anias into the family's new house for the first time with his older brother, Aza. Hide Caption 23 of 24 Photos: Conjoined twins separated: New life, apart Christian McDonald holds his son Anias as Nicole McDonald checks on Jadon at home with the family dogs, Taz and Tyson. Hide Caption 24 of 24

The boys' parents let CNN exclusively into their lives, to follow them through the marathon surgery and recovery, culminating in a documentary, "Separated: Saving the Twins."

The McDonalds were faced with an unimaginable choice: separate the boys and enter a world of unknowns or keep them together and likely watch them deteriorate and die.

Giving the boys 'their best shot'

Christian says they undoubtedly made the right choice. "If that's what the boys needed, then that's what we were going to do," he said. "We were going to give them their best shot, and we feel like we did that.

"We feel like our faith and our belief definitely carried us through."

Christian and Nicole McDonald show Jadon photos on a phone while he gets measured for a new protective helmet.

Added Nicole, "when we decided to share our story with the world, I did it with the intention of knowing that everyone would see a miracle."

The miracle of the boys seeing each other apart for the first time; of Mom and Dad holding their babies individually; of seeing the boys sleep in separate beds.

Nicole still fights back tears remembering those moments: "You always have to look at every single decision you make for both children and hope that the one that you made is the best thing that you did for them individually, because you are in charge of their life."

Before the surgery, any time the parents needed to move the boys, they had to carry them together in a carefully choreographed way.

Nicole and Christian transported their lives from their Midwestern roots to the Bronx in 2016 to be close to the medical staff at Montefiore in preparation for the surgery.

The entire journey has been "surreal," Nicole said.

After the operation, while the boys were undergoing rehab, the family moved to a home in upstate New York that Christian had renovated. About four months ago, desperate to be near relatives and longing for small-town life again, they moved to Indiana.

They picked a home on about 2 acres of land, down rural country roads and tucked between cornfields. They wanted a spot to give the family space, to let the boys grow and prosper and to allow their older brother, Aza, 5, to thrive on his own.

Jadon plays with letters of the alphabet during a physical therapy session.

"I had reached a point where I couldn't do it anymore," Nicole said. "I needed my mom. I needed just anyone who could come in my house and help me, because it was getting too hard.

"I needed to go home. The boys needed it, and Aza needed it."

All of that is on display as Aza flies around the house, hiding soft rubber letters for Jadon to pick up as if on a scavenger hunt. Jadon and Aza even ride the school bus together to attend day care.

"Jadon and Aza are both in school, and Anias goes to preschool one day a week," Christian said. "So we're trying to be a normal family."

When twins are joined at the head, one tends to be more dominant. In this case, Jadon was the dominant one, whose body worked overtime to keep both of them alive.

JUST WATCHED A 360° look inside the operating room during the boys' surgery Replay More Videos ... MUST WATCH A 360° look inside the operating room during the boys' surgery 06:16

Post-surgery, Anias has struggled more with breathing and an array of other issues. He used a breathing machine to keep him alive and other equipment was hooked up to him to monitor his vitals.

Anias faced major setbacks over the past year. He stopped rolling. He stopped playing with toys. He stopped lifting his head because so much fluid had built around his brain it weighed him down.

"It was horrific for me to watch," Nicole says. Anias had not "just stopped development, but regressed in development."

He also developed eczema so bad, his mother says, his legs and ankles were like "raw meat." He started having seizures so severe that he'd turn blue up to 15 times a day.

"I thought I was losing him forever, and I was devastated," Nicole said.

But he recently had his tonsils and adenoids removed, allowing him to breathe on his own and freeing him from the array of medical equipment that was hooked up to him. Since then, the parents have seen a vast improvement. He's begun lifting his head and using his hands and feet in ways he couldn't before -- a spark of hope after a year of heartache.

On the fridge, the family displays a hand-splattered painting by Anias, titled by his parents. "Painted this with the right hand that I'm not supposed to be able to use," it says.

"Anias had like Mount Everest medically," Nicole said. "That's what I've started to appreciate is, he climbed his medical mountain."

Christian said, "He's making progress. It's just slow. But it's there, every day."

'You just want him to fly'

When CNN last visited, the boys were closer in their progress, considering that the surgery had set them back to infancy in many ways. They had to learn how to sit up, crawl and do the other things every baby learns. Anias was a few months behind Jadon developmentally, but it seemed he was on track to catch up. He was able to roll and clap his hands.

Anias lost most of his function over the last year, but his parents say they've begun to see major progress in the past few weeks.

"What was hard was to see that he was on the same path as Jadon, and then something happened -- lots of things happened -- to pull him out of that race for a minute," Nicole said. "I feel like I'm constantly fighting for the Anias I know."

As a mom, she says, "you just want him to fly, and he's still stuck on the ground. That was really difficult, but we just had this ray of hope, like in the last month.

"That's lifts us both," she said, "and we're ready to help him take off.

"He's the most medically stable he's ever been. He's getting stronger every day," she said. "I think we're going to see a huge change in him over this next year."

She's determined to see Anias become the boy she believes he can be. She tells him things like "someday, you're going to sit up. Someday, you're going to walk to me." He often smiles at her encouraging words.

Both parents say they are focused in the moment, simply trying to enjoy their boys and relishing their growth.

Nicole says she will never stop advocating and fighting for her boys.

"The boys, they may not be perfectly normal little boys like you and me," Christian said. "But that doesn't mean they can't have a great life."

He says he and his wife used to worry about things like whether the boys' hair would grow back enough to cover the scars on their heads from the various surgeries. But now, they now intentionally cut their hair short: "They earned those scars. You know, they can be proud of them."

At the family table, Nicole recounts a story of Jadon spilling a drink at lunch and how he put her at ease, saying, "Mommy, it's just an accident." She says he then looked at her and said, "I am smart." She laughed and told him, "You are smart." Nicole was still laughing when Jadon added, "I am funny."

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As she tells this story, Jadon looks at her attentively. She bends down and kisses him on the top of his head. He decides to tack on third adjective to describe himself.

"And happy," he squeals.

"Jadon is happy," Nicole tells him.