I think this is a challenge - it would seem that a community with more traffic would find the idea not particularly interesting, while a lower traffic community would like it. The bigger community will always have less to gain and more to lose.

Major cross posting is also likely to generate duplicate content issues for search engines.

It depends on the objectives of the communities, I suppose. A couple of not-for-profit communities might better serve their overall goals by combining their effort in some way.

Thoughts:
- integrated search that scans both sites for matches
- shared user database that, for example, attaches a string to each user from the other forum (to prevent the inevitable conflicts) - for example, if I posted on ForumB, I might be ForumA-rogerd if I didn't actually register at ForumB.

I think site owners would not mind if this "aggregated" site simply used RSS feeds to combine all the posts... that way they are getting links and no dup content since you are basically just sharing titles and the first few sentences tops.

Thank you guys for your thoughts.

Can it be something like this. Both forum sites contain list of messages from each other. But when visitor, being on one forum, clicks a message which belongs to another forum, she is redirected to the forum the message belongs to.

May it work?

It could but you're looking inside the box.

Step back for a second.

Every forum is already linked together, its called google. Wether a forum is on your site or someone elses its still just a click away.

Check out your competition and brainstorm a way to do it better. Every detail matters, forum names, layout, appearance, choice of anchor text, even the flavor of your cemmentary matters a bit.

Try it out and see what happens.

cemmentary = commentary

/note to self : get more sleep