Good luck! Note though that JJ2 was never intended to support online cooperative play (that's why it's not available in the server setup screen), and while JJ2+ has done some work on some rough edges (clients getting hurt properly, for instance) there's still something to be desired about the interior. So after typing /coop or /gamemode coop, I think you still need to type /r once so that everything loads properly. (Or use their dropdown menu equivalents.) And more vexingly, enemy positions won't be synced among players.