Joined twins occur in about one in 100,000 pregnancies.
The twins come from a single fertilised egg, so they are always identical and the same sex.
Only around 19 pairs have been dealt with at UK hospitals since 1984.
There is always a debate over whether to separate joined twins, especially when it may mean that one twin will die.
The question that is often asked is: "Do we value one normal life more than two less able lives?"