Two forums share members with a twist

[vprp]

I've read a lot of people wanting to be able to share memberships within two different forums. It sounds interesting to me but with it, you'll probably get a lot of members who aren't interested in being a member of the other forum. So this is what I was thinking.

Let's say you have a forum with a user base and you're launching the new forum. You tell your members that they can sign up at the new forum but instead, give them the option to press a "Join" button that automatically registers them with their user account from the old forum on the new forum, too.

Would this be possible?