I was working on something for a customer and then got a brain storm.
http://sanguin.net (original site main for now)
http://darksanctum.net (same database, same files, same folders)
They break no rules of the one install policy (although both are at this time licenced)
Searches, Posts, Forums all work seperatly (and now new tables for them)
Private messages are global between the two sites as are post counts and so forth.
Thought I would get some input (I know the style on dark santum bites !!!)
Please do not sign up unless they are of interest (not after members) you can see by looking the functionality.