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.
Next is a design for the pillar mount.