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.