Get a tank with a commander's gun (a T-100 is a good example due to its low ammo count), ensure AI are manning the gunner and commander seat, get in the driver's seat
Select the coaxial gun for your AI in the gunner seat to use
Exhaust all the ammo in the commander weapon (you can have the AI do this themselves or do it yourself so before the next step)
Tell your AI in the gunner seat engage targets with their coaxial weapon while an AI is in the commander's seat
Observe the results
Move yourself into the Commander's seat/get the AI currently occupying it to jump out
Observe the results (the coaxial gun will begin working again)