Have you ever considered swapping sticky/pinned posts with other forums? For instance, both forums would pin an ad post about the other post on a related subforum and this post would be mutual advertising. And, then when anyone visits the related forum, they would return and post on that thread about their visit. Maybe this idea would work for your forum as a promotion idea to get new members. Would you use this?
It sounds like a nice idea, so long as both forums have a similar sort of focus, otherwise it's a little bit pointless. You could even have a sort of weekly competition thing that is on Forum A for one week and Forum B for the next, encouraging users to visit both.
