Maybe what you can do is setup domain1 as you normally do, then for domain2 you setup a redirect to:
http://www.domain2.com/forumdisplay.php?f=71 (or whatever the forum url is for domain2)
so domain2 is actually an alias for domain1 so you still have domain2.com in the url bar...
I'm sure you can also install VB on each domain and point to the same database, but then you may run into a licensing issue on that one.