At Admin Panel -> Settings -> Switches, the "secondary forums" switch enables submissions (of both topics and subforums) to multiple forums at once. With it on, you can submit one topic to several forums or one subforum to several parent forums. The submitter still chooses a primary forum first, then chooses secondary forums after that below.

Once you enable it, you'll probably want to configure the usergroup permissions at Admin Panel -> Members -> Manage Usergroups. There you can decide whether each group should be allowed to choose multiple forums, and also set how many they're allowed.