BALTIMORE -- Two 7-month-old West German boys joined at the head are to be separated at Johns Hopkins Hospital this week in a delicate operation that surgeons say they have no better than a 50-50 chance of surviving.

The Siamese twins flew to Dulles International Airport in Northern Virginia yesterday with their parents, Josef and Theresia Binder of Ulm, West Germany.

A team of 20 doctors will be on hand for the operation, which is tentatively scheduled for Saturday, said Lisa Hillman, director of public affairs for the Johns Hopkins Children's Center here.

The Binder twins are joined at the back of the head and share the major blood supply, but their nervous systems seem to function separately and they otherwise are doing well physically, Hopkins officials said.

"We're very hopeful their brains are separate," said Dr. Benjamin S. Carson, director of pediatric neurosurgery, who will be among those leading the team of physicians. "It's hard to be 100 percent sure. We won't know for sure until we get in there."

About 50 doctors, nurses and technicians are training together for the operation, expected to last from 12 to 14 hours, Hillman said.

The operation is only the second attempt at separating Siamese twins in the hospital's 98-year history. The first, five years ago, succeeded on infant girls from Dagsboro, Del., who were joined from the chest to the upper abdomen, Hillman said.

Carson estimated the frequency of Siamese births at about 1 per 2 million to 2.5 million, but he said the survival rate of twins who have been separated is hard to determine because such statistics are not updated.