If the best argument against doing something simple and logical is "Because, um...balance" Then there are larger problems in play.
Sorry, but if you're unable to accept game balance as something that needs to happen in said games because it's not how you would do thing in real life, then you should probably re-examine your point of view on games in general.
Please go back and read my post cause you missed the point. The point isn't what would I do in real life. It's what would I do in the guns of Icarus world, operating under the constraints of that world's accepted behaviors. For all the discussion I've seen here, I haven't seen a single reason given why an even marginally clever crew wouldn't try to make something like a double gat happen IN THIS GAME WORLD.
Aside from that, I think the arguments that it would be OP are overstated. Sure, it would be dangerous, but would a double gat on the front of a goldfish be any more dangerous than two front gats on a pyramidon? Which is worse on a spire, a gat, heavy flak combo, or a double gat, light flak combo? Personally, I'd still have to give it to a well timed gat heavy flak combo. More time to destroy the armor, but less chance of fixing it before the armor comes back up. Yes, a double gat and heavy flak combo on a galleon would be terrifying, but anyone who stands at medium to close range on a galleon's broadside has a death wish already. Of course, the only way to know would be to try he guns and see how they work in the game. Personally, I'd love he chance to play wit hem and see.