Doctors said yesterday they were "slightly encouraged" by the condition of 7-month-old Siamese twins whose heads were separated in delicate surgery Sunday, but they stressed that if the boys survive the first few days it will still be weeks before they can predict the future health of either child.

Dr. Mark Rogers, director of pediatric intensive care at the Johns Hopkins Medical Institutions, who reviewed the twins' condition yesterday, said he was "slightly encouraged they are doing well."

Hopkins officals had given the twins a 50-50 chance of surviving the 22-hour operation in which doctors separated a shared blood vessel system in the back of their brains. The boys, Patrick and Benjamin Binder of Ulm, West Germany, were in critical but stable condition as they rested in separate cribs for the first time since their birth Feb. 2.

Doctors said the twins, who will remain in an artificially induced coma for five to seven days, must undergo more surgery to reconstruct the rear portion of their skulls. One twin, Patrick, had enough skin to cover the opening in the skull, but the rear of Benjamin's scalp had to be covered with fabric mesh.

"We believe they have been successfully separated. But we would prefer success to be {seeing} the Binder twins return home entirely healthy and normal," said Rogers. "We are not now in the position to guarantee that in any way, shape or form. They are far from out of the woods."

"We are very gratified that they survived surgery," said Rogers, noting that surgeons, to control bleeding, had to lower the infants' body temperature sharply and deliberately stop their hearts before their heads could be separated.

The parents of the boys, Josef, 36, and Theresia, 20, will not talk to reporters or release photographs of their sons because they have a contract with the West German photo magazine Bunte.

"Needless to say, like any other parents, they were greatly relieved," Rogers said after the lengthy surgery.

Siamese twins -- newborns joined in some manner -- occur once in every 70,000 to 100,000 live births, doctors said. Twins are joined at the head approximately once in every 2 million live births. Siamese twins form because a single fertilized egg fails to divide completely.