There is no support for such an usecase yet. It is planed but not in very near future. Simply, there are bigger priorities for now, sorry.
If it is urgent for you, then here are two ideas.
If you set up different database schemas for each forums deployment, you would have to create sql views for Portal 's User table(s) because Forums use them to get needed info about user. I have no idea if this would work with hibernate, I have never tried such a weird configuration ;-).
You could contribute a patch for Forums which would display forum categories or forums as specified in portlet instance preferences. It shouldn't be very hard to code. Then you could create different portlet instances for every page you need and add appropriate preferences to constrain categories/forums. This seems to be the most reasonable solution.
If it is not so urgent then you can wait till I'll do it some day.
JBoss Forums Lead
JBoss Labs Team