Read this:
FIRESTIK INFOThe second advantage of co-phased antennas is there ability to perform where there is little reflective ground plane for the single antenna to radiate its energy from. For instance, on fiberglass vehicles or those with light aluminum sheeting over a non-metallic frame. In those situations a co-phase set-up allows the antennas to use each other's radiation field to direct the combined energy across the horizon. In order to be effective at least 2/3's of each antenna must have unobstructed line-of-sight to the other antenna.