Official comment
@Adam > You don't need a hub if you're only linking two players; they can be connected directly to each-other via ethernet cable on the ProDJ Link. Unfortunately, the CDJ-900NEXUS doesn't support the USB-to-ProDJ Link bridge, and you'll need to either connect both computers via USB (for using rekordbox in PERFORMANCE mode), or ethernet (for using rekordbox in EXPORT mode), in which case you would need an ethernet hub, as well as an ethernet adapter for your MacBook.