Twins Who Share A Brain Tatiana and Krista Hogan are craniopagus twins (Tatiana is on the left). They are connected at the head and share a brain. They were born Oct. 25, 2006, to parents Felicia and Brendan Hogan of British Columbia, Canada. "These twins, I think to our current knowledge, are the only ones that do have a common neurological connection," said Dr. Doug Cochrane, the twins' pediatric neurologist. Courtesy Renegade Pictures

Twins Who Share A Brain Tatiana (left) and Krista Hogan at home in British Columbia, Canada, with mom Felicia Hogan, left, and grandmother Louise McKay. "[The doctor] just sat there and he's like, 'Um, there's no way to say it, but your girls are conjoined, and they're conjoined at the head,'" recalled Felicia Hogan. "And he's like, 'I have no idea if these girls are going to survive. ... You still have time to abort them.' And right then I looked at him and said, 'There's no way that's ever going to happen.'" The twins were born healthy and strong, weighing 12.5 pounds. Courtesy Renegade Pictures

Twins Who Share A Brain While Tatiana, left, and Krista move together amazingly well, they don't always agree on which way to go or what to do -- and that can cause problems. "[Krista will] just get so frustrated with her sister that she'll -- the only way that she knows how to get it out is to scratch her sister," mom Felicia Hogan said. "We try and get her to stop, but usually we're 10 seconds too late." Courtesy Renegade Pictures

Twins Who Share A Brain The twins' happiness is apparent to all around them -- they laughed and played the entire day the "Nightline" crew was with them. "They ... have this connection between their, what's called the thalamus, between the thalami, one in each to the other," said Dr. Doug Cochrane, the twins' pediatric neurologist. "So there's actually a bridge of neural tissues in these twins, which makes them quite unique." It also makes them impossible to separate. Mom Felicia Hogan and others believe the connection has given the twins unique powers. "They share a lot of things normal conjoined twins don't," she said. "They have special abilities to see each other, see what each other's seeing through each other's eyes." Courtesy Renegade Pictures

Twins Who Share A Brain The twins' condition has given them very real problems. Tatiana, the smaller of the two, has an enlarged heart and actually pumps much of Krista's blood to her brain. The complications have led to surgeries and more worries. "We just kind of live today," said mom Felicia Hogan. "We live within today. Forget about tomorrow, we'll deal with tomorrow when tomorrow gets here." For now, the girls are healthy and strong. Courtesy Renegade Pictures