I looked into something similar for my daily bicycle habit. THe maker of one bike tray said to absolutely NOT use this sort of thing. He pointed out that the tongue weight of two bikes and the bike tray easily outweighe the maximum alloted by most of these bumper bolted hitch mounts. The hitch needs to be bolted to the car from the frame underneath some how.
So basically forget about hooking this up to a tire tail.
I decided to go with this set up ($400)