I am currently running 1 smf forum and I am consider running a second forum on the same package (shared hosting) under another domain, but what I am wondering is if it will be too much load on the server, as both forums will use the same mysql database (the domain is just masking).
It really depends on the sizes of the forums and if your host limits resource usage that way.
So just in general having two forums wouldn't generate a lot of server load...
Like i said, it depends on size and activity.
its possible for a forum to use more than a single server even. Its related to the size and activity, just like any site.