I considered writing about it for this topic actually, because I've always wondered how things would have been. It's something that's made me unhappy all my life as I've felt the loss, as odd as it might seem, and I've hated being an only (living) child. It seems like it would be such a woderful relationship!
I have known one or two sets that split, and refuse to interact. I guess people will be people, twins or not. The ones I've known very close and had great relatioships even as kids (I went to grade school with a set of boy twins who even had their own code to talk in!). That's so sad, but so very true what you said about people being people whether they have a twin or not! :/