The problem is that existing users on the existing forums wont be able to automatically participate there. They'd have to register again.
We were considering a multi-site plugin, but I was wondering - it would be a lot easier if we could do this:
1) Copy all existing users into the new XF install so at least current users have access.
2) Add something to the registration screen of the native forums which could be a simple checkbox, saying "Yes, please also register me on the other Forums"
3) Checking this box acts to register them on the other install as well.
Seems a lot easier than having to manage some multisite AddOn that might break, become outdated, etc.
This seems a lot more basic, and just a one-time modification.