There is no actual technical problem with the over and under barrel configuration. They would have to be packed tightly together at the breech. But to be honest?
The game's resolution breaks down when you look too closely at the function of in-game things like these guns. You are below the 30-meter limit and can see the individual pixels, so to speak. Arguing whether those pixels should be grey or black falls into the realm of opinion. At the scales we're talking, it really doesn't matter whether the guns would throw each other off from recoil, whether there is a possible compensating system for the barrel movement, or what - as long as the idea itself isn't TOO jarring, it is, as Eriance might say 'all good'.
And that is very much what Eriance means by 'not bothering with the physics'. He DOES bother with the physics - that is why his tanks appeal on so many levels. However... he is applying the physics on a much larger scale than we're discussing here.
Steve