[Rupert]

Consistency with the rest of the rules would make such a grouping work like any others - they are all lumped together in one attack, with a rate of fire equal to the total RoF of all the guns added together. See SS, p.57-58 "Rate of Fire".

I don't see any reason why multiple major batteries can't be grouped together like this. Then again, I'm cool with ships being designed with some or all of a medium (or smaller) battery's guns being placed into a turret and all being fired together. Naturally such a set-up would prohibit those guns from engaging separate targets.