In the main pilots seat it seems you can only control one. I might be wrong, sorry.
Is okay
But yes I have to say youre wrong, there are 2 seats, you can change with the , and the . (the same buttons as any vehicle), to change the cannos you use these buttons:
2 (non numpad): M61 Vulcan Cannon (Machine gun)
3 (non numpad): 105mm M102 Howitzer
4 (non numpad): L/60 Bofors Cannon
But if you don't have any brick, it wont work, you have to actually have bricks in these spaces for it to work