Just make sure you have your cookies set to forum and not site (in admion options), and in the config.asp file, change the table prefix for your additional forum to something other than your first forum.
You don't necessarily have to set your cookies to the forums, if you want to set them to website, you could do so but make sure you change the cookie name by changing the value of strUniqueID in config.asp.
Search is your friend “I was having a mildly paranoid day, mostly due to the fact that the mad priest lady from over the river had taken to nailing weasels to my front door again.”