Had a chat with Blizz a few days ago about this, the ONLY reason it is "allowed" to happen is differences in names, (i.e. his example was William, Bill, Billy, Will, Willy and so on). The rule is very clear that is is not supported between accounts you do not own. (see the forums to understand what I mean by own, I mean legit Blizz own; all names match and so on.)