Don't do it. The thin metal tube bumpers have a tough time with loads as low 75#. They will twist and break the welds with loss or damage to everything that is attached. In addition, if they fall off your vehicle and cause an accident, you will be liable if you are caught.
You're much better off purchasing an aftermarket frame mounted hitch that will meet your needs. For example, the one linked below has a capacity of 500#. You could use it to mount the bikes, and then move your spare tire under the trailer with a frame mounted spare tire carrier.
Frame Mounted RV HitchJim
