Siamese twins, sharing a heart and liver, have been born in the UK. see http://news.bbc.co.uk/hi/english/health/newsid_1959000/1959294.stm
They were discovered during the 14 week scan and the parents offered a termination, which they declined. Now that the twins have been born they have to be separated as they will shortly die if they are not. One of them will certainly die and one is considerably weeker than the other. The chances for the remaining one is also slim.
It's going to cost a lot of money to look after them and it will be paid for by the National Health Service. I personally feel that it was unfair of the parents to allow the pregnancy to continue, given the chances of survival, the suffering that will be caused to the babies and the cost of treatment when some many other patients could be treated in their place.