Two Iranian sisters joined at the head are ignoring their doctors' advice and undergoing an unprecedented and highly risky operation this weekend in Singapore to separate them.

Law graduates Ladan and Laleh Bijani, 29, from Shiraz, are by far the oldest people joined at the head to undergo the complicated surgery; experts say at least one of them may not survive.

But the women sounded remarkably upbeat in a statement released by Raffles hospital, where the operation will take place. "We've been praying every day for our operation. We are excited as we've waited 29 years for it," they said. "Both of us have started on this journey together and we hope that the operation will bring us to the end of this difficult path, and we may begin our new and wonderful lives as two separate persons."

They have told doctors the first thing they want to see after they come round is each other's face, something that has only been possible until now by using mirrors.

The neurosurgeon who will lead the team, Keith Goh, says he has tried to talk the twins out of the operation, although it could succeed because they have anatomically separate brains.

They were rejected seven years ago by German doctors, who said the risks were too high. The surgery, during which the twins will be seated, is expected to take at least 48 hours. When Dr Goh led a team that separated Nepali infant girls Jamuna and Ganga Shrestha in 2001, the operation lasted over 100 hours. They are still alive.