I have to question that assumption.
Yes, there are cases where, unequivocally, one twin must die or both will die, but cases like Abby and Brittany Hensel indicate that conjoined twins can grow up healthy and active without separation. There are other, older conjoined twins who say they do not wish to be separated and are glad they are not separated.
A difficult question.
I do hope the doctors and parents involved here would consider the Hensel twins and alternatives to simply sawing a body in half or lopping off a head.