With a 17-hour operation and its one nasty surprise behind them, the surgeons who separated 2-year-old conjoined twins said Thursday that it will be days before they can see how the boys respond to their transformation.

As the doctors spoke at a news conference at Montefiore Medical Center (search) in the Bronx, Carl and Clarence Aguirre, who had been connected at the tops of their heads until Wednesday night, slept the day away upstairs, side by side instead of head to head.

Louis Singer, chief of pediatric intensive care at Montefiore's Children's Hospital (search), said the boys, who are from the Philippines, probably would be kept sedated through the weekend to help them heal. Doctors earlier had said the boys were expected to awaken Thursday.

The hospital continued to describe the boys' condition as "strong and stable," and its president, Dr. Spencer Foreman, declared the separation a success.

Doctors performed four major surgeries since October to gradually separate the boys, instead of the marathon sessions used in previous separations of conjoined twins.

"A huge step forward in this remarkable process has been accomplished," Foreman said. "We hope that Carl and Clarence will now begin to thrive as two separate and unique individuals."

Dr. James Goodrich, the neurosurgeon who led the operating team, said surgeons discovered that a 2-square-inch area of the boys' brains at the back of the heads was fused just as they thought they were nearing separation.

Doctors had expected the brains to be abutting, but they were "anatomically linked," Goodrich said at a hospital news conference.

"Obviously the brains were talking to each other," he said, meaning working jointly in some way.

By referring to three-dimensional images created in advance and following the contours they could see, the doctors manipulated the brains apart without resorting to an incision, Goodrich said. But he said fused brains cannot be separated "without some consequence."

"The hope is that the consequence is minimum at best," he said.

Goodrich said it would be "at least a week to 10 days before we can really assess what changes have taken place." He said he was encouraged that brain swelling and blood loss were minimal, often a sign that brain damage has been avoided.

He said twins joined at the head have never been separated without at least one suffering brain damage.

He added, "Would I do it this way again? Absolutely yes."

After the separation, Goodrich and the boys' plastic surgeon, Dr. David Staffenberg, started covering the brains, leaving reconstruction of the skulls for later.

The coverings included some oddities -- a material made from pigs' intestines was used to augment a membrane called the dura, and part of Clarence's scalp and hair was placed atop Carl's head and vice versa, Staffenberg said.

Goodrich and Staffenberg said they will be fascinated to see how Carl and Clarence react to the sight of each other. Doctors used mirrors and Velcro-headed dolls in their efforts to prepare the boys.

"There has to be enormous psychological import," Goodrich said. "But obviously they can't tell us."

Goodrich said the staff will be on the alert for any infection in the boys, leaks of spinal fluid and accumulations of liquid on the brain.

Staffenberg said that when the boys have healed enough, they'll go back into physical therapy at Blythedale Children's Hospital (search) in Valhalla, where they and their mother, Arlene Aguirre, have been living between surgeries at Montefiore. Both hospitals and the doctors have donated their services.

Staffenberg said the rebuilding of the skulls will be done in two or three stages. It will be delightful, he said, to have "Carl and Clarence in the operating room as separate little boys."