I'd never thought about why they'd do it, but this makes perfect sense.It would work best when the antennas are exactly a quarter wavelength apart, but a few inches won't matter much.

It's possible to get two antennas a quarter wavelength or more apart to direct the best signal strength in any direction desired with the right phasing harness in between, but this is probably much more trouble than it's worth for a CB.

 

duals! (:

 

well, i think im going to go with a 3 or 4 foot Single Antenna in the middle of my truck, up grade later, comes with some gray coax cable, and the mounts, and the antenna, if i ever want a bigger antenna i could go buy another =)

 

I think you'll find that does just fine.

 

Anything less than a 1/4 wave (102") whip is a compromise. Base & center loaded coils, or wound antennas, are the proper 'electrical' length but not the correct 'physical' length. They radiate fine but reception will always be inferior to a full sized antenna.
As someone stated before, antennas placed too close together will de-couple, partially canceling each other out. Auto bodies and truck cabs do the same thing. Bumper mounts are also a compromise, even with a single antenna, because its too close to other metal objects. (It seems contrary since it still uses the vehicle as a ground plane radial, but its true non the less). Thats why semis have dual antennas, the cab blocks the signal to one side, just like a reflector element on a Yagi antenna. A reflector doesn't really reflect by the way, it creates a a null to the rear simulating improved reception from the front.