By Stephanie Stahl

PHILADELPHIA (CBS) — New details are emerging about the historic surgery on the conjoined twins at Children’s Hospital of Philadelphia.

Abby and Erin Delaney were separated four months ago and are doing well.

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“The rapid growth in these girls and what they’re doing. They’re saying ‘da-da’ and sitting up, it’s really been remarkable,” said neurosurgeon Dr. Greg Heuer.

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Heuer says the girls will need one more surgery to finish reconstructing their skulls.

“We’re going to have to go back in there and put some synthetic material to give them the shape of skull that we want and to be more protective as well,” said Heuer.

Heuer has been taking care of the twins and planning the intricate operation since they were born 15 months ago at CHOP.

The separation surgery on June 6 took 11 hours, a harrowing time for parents Heather and Riley Delaney, who are from North Carolina.

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Before the big surgery, Heuer says there were a series of smaller operations – first a cut where the heads would be separated.

“Then, externally, we put a device on their skull, pushing each twin further apart,” explained Heuer.

This is the first time CHOP had used the external distraction device.

“This is custom-built on a 3-D for them based on their scans,” said Heuer.

Now, the twins have recovered enough for mom to play dress up. Doctors are giving an optimistic prognosis.

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“They’re such happy, content little girls for what they’ve gone through,” said Heather Riley.

“The possibilities are still completely open for them and they’re going to turn out perfect for them,” said Heuer.

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Abby is still getting some therapy at the hospital, while Erin has been discharged. The plan now is for the family to be home together for Thanksgiving.

Stephanie Stahl