Can this unit (the XDJ-AERO) and another DJ device (e.g. CDJ-2000 or DJM-2000) be connected simultaneously to rekordbox?

No. This unit (the XDJ-AERO) cannot communicate with rekordbox (Mac/Windows) or rekordbox (iOS/Android) connected to another DJ device (e.g. CDJ-2000 or DJM-2000).