As an electronics engineer, with experience in communications, and a ham radio operator, I have to agree with AllanD.
It is much more effective to tune two antenna elements to reinforce each other linearly, than to attempt to phase them in such a way as to peak in a sideways direction. I mean, look at a TV broadcast antenna, or ham radio multi-element "beam" antennas...all the gain is in line with the elements, not off to the sides.
If you want a slightly stronger signal front-to back, go for duals, but mount one on the fender and one on the bed rail. Just a hair over 9 feet apart is great. Then run two pieces of coax, exactly the same length (and cut from the same cable, if possible) to a T-connector, and then run one cable into the radio.
Side-by-side duals are likely to be more responsive to the sides of your vehicle, and have weaker reception front-to-back, than a single-antenna setup, without some complicated cabling and mounting arrangements. And again, you'ld want 'em 9 feet apart, which isn't likely to happen on a compact pickup.
Most users will find the best performance is realized with a single antenna, mounted centrally on the vehicle. Even if it doesn't look as "tough".
Spott