Yes you can install 2 testboards. I personally have 2 testboards for each version, which means I have loads of boards installed
If you are starting from an empty forum and want it to share the user database with another board, that isn't particularly difficult to acheive. Basically you need to go through each file that deals with users on one of the forums (ie member.php, usercp.php, global.php, sessions.php, etc) and adjust the queries to the database so that they query the other forum's user table rather than it's own user table.