i have two ipb forums (etc.: a and b) and i want to unit these forums. So, a member who is register a forum is would write new topic in b forum .
How to do this?

Share the users table?

I think what he wants is what converge is going to "bring". if say I register here (which I did), I would also (example[ly]) register on beyond.

This will be possible with converge which *should* be realeased with IPB 2.2.