Here is the result of my dual antenna mount for the spare tire bracket. At the moment I have a marine and 2-meter antennas mounted.
The vertical brackets were made for me by Arizona Rocky Road by extending their spare tire bracket mounts to 20 inches in length. Then I made a pattern for the dog bone top plate for the mag mount antennas. My friendly machine shop made the plate which was bolted onto the vertical posts.
The GAM Electronics SS-2 antennas were screwed into their mag antenna mounts which stick onto the dog bone plate. When I am in the city and need to use the parking garage, I pull the mag mounts off the dog bone plate and put them in the back of the JK. This way it hides the fact that there might be radios onboard.