the
Daily Mail has the full story of these guys, identical twins born at the same time. It turns out to be a case of
twin to twin transfusion, where one twin sends blood to the other through a common placenta.
In this case the problem was detected at 15 weeks, so early that the second very-medical link doesn't even put it in the category of 'severe'. It's a complication that endangers both twins; the donor twin is usually anemic, and the recipient has excess red blood cells and jaundice. But the story says both have shown a will to live, even at their tiny age and size.