If separation were possible --- as, sadly, it clearly wasn't in this case --- it would be very hard to argue against it.  That would be a very difficult life.
Due to Original Sin, once in a great while, twins are born in this condition.  It's very unfortunate but it can't be helped.  It's probably a foregone conclusion that anyone who admits of the moral licitness of abortion, would advocate for it when twins are conjoined and could not possibly be separated.