At Admin Panel -> Settings -> Switches, the "secondary categories" switch enables submissions (of both listings and subcategories) to multiple categories at once. With it on, you can submit one listing to several categories or one subcategory to several parent categories. The submitter still chooses a primary category first, then chooses secondary categories 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 categories, and also set how many they're allowed.