[Andreas]

I think this is not too difficult, at first sight (just some brainstorming):

Go through all PHP-files and put constant TABLE_PREFIX before all table-references except tables user, userfield and useractivation (you could use a different constant for them).

Install both forums using the same database but different TABLE_PREFIX (i guess after the first install you will have to delete the tables user, userfield and useractivation otherwise the install-script will cry )

BUT: This causes some major problems like post counters, usergroup-memberships, last post/activity, etc.