[One Country Boy]

I don't have a lot of experience with phasing antennas. I know the length of the phasing harness is taken into consideration, depending on the band you are operating. Since you would be using multiple bands with the Diamond CR8900A that may be of concern.

You mentioned that you were attempting to get a good 360 degree radiation pattern. I think you would be defeating this purpose by phasing the antennas. If you're mounting the antennas on the outside mirrors the radiation should be minimal to the sides of the vehicle. The maximum radiation of the pattern would be to the front and rear of the vehicle. Phasing, co-phasing or using them as dual antennas, in my opinion, would not be an option when attempting to create a good 360 degree pattern.

Those Diamond antennas are top of the line antennas. I've never used the CR8900A but once ran a dual band Diamond (2 meters & 440) on the tool box of my truck with outstanding results. It was about 5 or 6 ft in length until I drove through a parking garage with it.

As "Radio" mentioned, your best bet, to achieve a good 360 pattern would be to mount one antenna. I hope this has been of some help other members may have other ideas.