NEW YORK (CNN Newsource) — On Thursday, conjoined twins will get a chance at separate lives.

The risky surgery started on Thursday morning at Montefiore Medical Center in the Bronx and could last up to 20 hours.

Their parents talked exclusively to CNN’s doctor Sanjay Gupta about the procedure.

“We had the boys, of course, and they were normal little boys, just like two, any other two little babies you would see…except for being conjoined,” father Christian McDonald said.

“I could almost just keep them like this right now because they’re so perfect,” mother Nicole McDonald said.

The two boys are named Jadon and Anias.

Jadon is the more rambunctious one. Anias is quiet and inquisitive.

He’s also had to struggle more.

“He’s really beared the brunt of the burden, I would say,” Nicole said. “He’s got breathing issues, and feeding issues, and he has some vision issues. In the beginning, his hearing was off. He’s gone through heat failures. He’s had seizures. He has had so much happen in the last year, and he…just rolls right off his back. He just keeps on going,” Nicole said.

Anias and Jadon McDonald’s birth was exceedingly rare.

Twins joined at the head, called craniopagus twins, occur in 1 out of every 2.5 million births.

They rarely survive past the age of two if not separated.