That doesn't work well on a Jeep. The co-phased antennas you are asking about and often seen on big trucks work only because they can be spaced far enough apart for it to help with forward and reverse range. A Jeep is too narrow for that to work. However, that co-phased setup also ruins the omni-directionality of a CB antenna which means its range off to the side is reduced.
Personally, I would not even consider it.