Thule Rak-N-Loc hitch mount rack. Includes 2-bike add on module and two Thule locks with matching key.
One lock secures the hitch pin to the vehicle and the other secures a heavy-duty, swing-down latch that secures the bike frames to the rack. Also has a piston-driven latch that allows it to tilt away for better hatch/tailgate access.
More attractive examples go for $125+. Mine is not pretty but gets the job done. $30 OBO