For example, if I had two websites catering to different groups of people, but I wanted the classified advertising section to be the same for both sites. So the sub forum is viewable on each respective site, but the user doesn't have to visit or even necessarily realise there's a another main forum elsewhere that can also access it.
This is prevent users who want to reach each target audience from having to create and maintain duplicate threads on each site.
I'm considering using something like XenSSO to enable single sign on for both sites so there wouldn't be duplicate users.
I'm guessing this might not be possible, or eat least very easy, for a number of reasons, but I'd love to hear what you guys have to say about it.