I like taking a myrmadon with a subach on the 4 gun mount where it does quite a big of rapid fire damage, and I'll put a prom R on the two gun mount and only use it for sniping bombs and turrets at max range. The subach energy drain allows me to up the power to shields and engines, making for a very powerful air superiority fighter. With this configuration, I'll usually load out with tempest dumbfires for extra shreddage at close range. I replace the prom R with any other long range gun I can take if its available. It really does suck.
I rarely double up on the same gun. I'd rather flip from a subach to something longer range or more damaging for brief salvos. Relying more on the subach allows me to crank the engines and shields. Using secondaries after shooting off the rear shield with a subach makes for fast kills too.