I think it's 'cause one person doesn't want an account but the other person isn't okay with not having that person's name somewhere on their page, so they put their names together. At least, I think that for the one I had. But now there are two separate accounts. Only one ever gets on, but they're officially married on facebook. :nod:
I always assume the joint accounters are those who don't get on FB that often and use it only as a tool to stay in touch with close friends and family. Who knows... I don't have any friends like that.