For the life of me, I have never found the specific rule in the ASL RB on firing both the MA and SA during the same fire phase. Maybe it was assumed and I'm a little slow on the draw.
I think the flow of assumptions is in the opposite direction: any weapon may be fired (a somewhat implicit axiom) unless you find a rule that says it can't. So for instance Personnel may possess any number of Guns/SW [A4.43], but a squad can only fire a maximum of two per phase [A7.351], and halfsquads/single SMC/team of two SMC may fire only one. As I said, vehicles can fire all their weapons unless you can find a reason they can't.
Really, a chain of reasoning showing that a vehicle may fire its MA is a bit elusive. Perhaps you might start with D3, "Most of the mechanics for AFV combat are covered by the rules of ordnance in Chapter C," and proceed from there. You might include C2.1, then C3.1, but there's really not a clear line of logic that says, "an attack may be made against enemy units by firing ordnance, including ordnance mounted on vehicles." To a certain extent it's assumed that that is why you are here, and you know that. Now perhaps there are other rules I have missed that make the logic clearer, but the rules are not written so that a space alien which uses only telepathic attacks and has nothing even slightly related to ordnance will understand. There is a certain amount of assumed knowledge built in.
All that weaponry gives these vehicles potent firepower.
I think the value of placing additional weapons on vehicles falls off fairly quickly. Experience proved that a vehicle with one major weapon system was a better use of resources than one with many independent systems (as with the T-35). I don't think ASL goes to any lengths to simulate this aspect directly, as the vehicles in question have enough other weaknesses that it's not worth the effort to add more debilitations. The American M3 Medium was not designed with the land-battleship prototype in mind. It was more a kludge to get a 75mm gun into the field even if it couldn't be mounted on a turret yet, i.e. it was of a different class from the land battleships of the inter-war years.
JR