In British Law, at least, there are no such examples of the scenario given and, therefore, should it arrive, the legal baseline for such a circumstance would rely on a Judge and/or jury. Until then, it's a legal anomaly.
If you cast your mind back to 2000, the European Court of Appeal had a significant landmark regarding Siamese twins and the law. The two twins were conjoined at the hip and doctors said they should be separated, or both would face certain death, however, in doing so, one of the twins would definately die. The parents did not want to go ahead, but, in the laws of ethics, the judgement had to be made.
This is about as close as we've come to the example of the thread. British Law is unique, and arguabaly flawed, in that it isn't bound by a written constitution, as with American Law, to act as a baseline. For example, little Johnny steals an apple to feed his starving son and is prosecuted successfully. That is the baseline. however, a judge, albeit very difficultly, rule that little Timmy, who did the same, would be innocent of theft - that then becomes the new baseline. This is a horrendously simplified version of the law, I concede, but it serves an example.
In relation to this thread, especially over such a contentious issue, I'd imagine in British Law it would really depend on what kind of Judge the twins are dealing with!