It’s the rarest form of a rare condition – conjoined twins, fused at the head.
The BBC has been granted exclusive access to a ground-breaking series of operations in a British hospital to separate two sisters.
There is a crowd in the operating theatre. But the team of nearly 20 works as one.
Moving smoothly, every motion calculated. No signs of stress or tension, just hands methodically performing tasks.
But this is no routine operation. The shrouded shape of two small girls is picked out by the bright theatre lights. Safa and Marwa are joined at the head. Their brains are exposed as the surgeons work to separate a labyrinth of shared blood vessels.
But then the calm and peace of the theatre disappears, as anaesthetists raise the alarm.
The blood from Safa’s brain isn’t draining properly and she is shunting blood to her sister.
This puts strain on Marwa’s heart and she becomes dangerously unstable.
The anaesthetists call out commands, report vital signs and busy themselves trying to stabilise the girls.
"I think we need to shock," says one of them.
Pads are attached to Marwa’s chest in preparation.
The lead surgeon raises both hands so they can be clearly seen, and steps back.
Then everyone waits.
If they lose her, they may also lose Safa./BBC news