Erin and Abby Delaney's parents are grateful their twin daughters are thriving. Born as conjoined twins, their condition was among the rarest of the rare. Now doctors are providing an update on the girls' extraordinary journey.

"They're just growing and changing and amazing little people and I can say they are really my heroes for what they have been through," Heather Delaney told CBS News.

The sisters were born joined at the head, and even rarer, totally fused with their connection deep into the brain tissue. In June 2017, the conjoined twins were fully separated at 10 months old at Children's Hospital of Philadelphia.

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"Around 3 months of age we separated the bone that was connecting the two twins and then we sort of slowly pushed them apart and changed the anatomy which, where the two were connected and then that allowed us to do the separation," explained Dr. Gregory Heuer of the Children's Hospital of Philadelphia.

Erin and Abby Delaney are now 2 years old. CBS News

A new report published in the New England Journal of Medicine details how doctors used innovative technology and teamwork to perform the complex separation surgery. A computer navigation system helped them map the connected blood vessels that needed to be divided between the twins.

"The most difficult part for these girls was that they shared some really important big blood vessels so having to be able to separate those and having the brain recover after we did the separation was really the hardest part," Heuer said.

The girls are among the youngest twins joined at the head to be successfully separated.

Now 2 years old, Erin and Abby are receiving physical, occupational, and speech therapy to aid their development. In the next few years, the sisters will need additional surgery to close the openings in their skulls.

"Some of the stuff they were doing had never been done before so we didn't know how it was going to work out," Heather Delaney said. "Luckily everything turned out incredible. We have miracle little girls to show for it."