When I consider what our family has done to keep both twins, through TTTS, through preeclapsia, the countless requests for Divine intervention and prayer (http://www.freerepublic.com/focus/chat/2627233/posts?page=303), the thought of destroying, of murdering the other baby or babies in the womb is a horrorshow.
Consider if for only a second that those babies are aware of each other in there (if you don't believe that, just suppose) and consider the abject horror for the survivor...
What parent would do this to their children?
I've got a niece and nephew who are twins, it is amazing how "aware" of each other they are. Even when they bicker it is totally different from normal sibling squabbles.
My brother's sister-in-law lost a twin during pregnancy. When the surviving twin was old enough they told him about his twin, he said that he had always known that someone was missing.