i've done that actually for a paid client (very generous i have to state).. .but for forums that shows on some other places... the forum is mainly the "Support Forum" for all the parts of the large community, and it is showing on a set of forums, like the other sub-forums...
basically it would be the exact same thing. you have ThreadID-X that have to be listed in forumID-X and forumID-Y, but not duplicated.. the same thread...
can be done...