If you are running two radios, then keeping them on other sides of the vehicle would probably be a good idea, but I'm not familiar with that.
If you're planning to "co-phase" them, and drive both with the same radio, save yourself the money. To do it right, they need to be 9' apart (1/4 wave).
I was looking into this for my ram, and had a nice long chat with an co-worker that does magnetic field work and used to be a big CB'r. So unless you've got a big-rig, you really won't gain anything.
Any when running co-phased, you are trading broadcast ability on the sides for broadcast ability front-rear. So while you can double the range to the front/rear, you lose it ALL on the sides... Fine for truckers in Kansas, but not so good for trail-use. So that's another thing to keep in mind.