Please take a look at these sites for example to get an idea:
http://www.m3post.com/forums/
http://www.e90post.com/forums/
You will notice these are two separate domains with two different sets of forums EXCEPT the universal forums listed at the bottom. The who's online section lists all the users online on either domain and even the other domains as well.
I think that's what you just mentioned.. If I have read it correctly.
Regards.