Shared dreaming is one of those things that you have to experience before you really can believe it, and since I haven't had one I'm slightly skeptic too, but I don't rule out the possibility of them.

I have a suggestion of what may have happened in some of the cases of shared dreams, but it would only be true for more common dreams. When I dream I dream about something I have done the same day, the days before or something I worry about that's coming soon almost every night, at least to some extent. For example 3 days ago I saw Ocean's Twelve and National Treasure on the same day. During one of my dreams that night I was Brad Pitt and my best friend was Nicholas Cage's nervous partner. So if you live very close with someone and you do the same things day in and day out chances are that eventually you will have a dream very similar to them. I don't know how often people talk about their dreams really. If you usually don't do it very often and then one morning ask the other to write down their dream if they had a dream about you, and you then write down your dream and you then exchange papers and they are the same it would probably be a shared dream. On the other hand if you always talk about dreams and then the 500th dream you talk about is very close to what the other one experienced I would call it coincidence.