Duel antennas are a PIA. Tuning is difficult and for them to work as intended, they need to be 1/4 (108.6") wave length apart. That generally puts them way out on your mirrors and in newer trucks, its becoming harder to find mountable real estate out there. Any distances other then a 1/4 wave length antennae spread and you are defeating the purpose and degrading the signal.

The purpose of duel antennae installation is to increase your power forward and to the rear. but you pay for this by decreasing side projecting power. Your Xmitter becomes very directional. And more unreliable.

With one antennae, if you encounter a problem, at least you know where the problem is not.