What if they were conjoined twins who couldn't have previously been separated without killing both? *edit Or, in less cutesy terms, considering that much of t

What if they were conjoined twins who couldn't have previously been separated without killing both? *edit Or, in less cutesy terms, considering that much of the pro/anti-abortion argument (on both sides) often rests on tacit and irreconcilable definitions of what a person is, the point where the woman's body ends and another begins, when is it appropriate (other than perhaps under the most obvious of circumstances) to charge and assailant that there were two homicides, rather than one? That is, when does the definition become more than personal belief?