As I stated before, the biggest problem lies in gunners being able to switch seats. If you have more than one gunner, the possibility to collide moving into the same seat presents itself and there's no desirable solution to that. You can have one gunner that can move about, or multiple gunners locked in their seats. Means you need at least four gunners to protect an aircraft from all angles.
Air Warrior (the early home of HiTech) had a message that the position was full. I do not remember if the gunner position required a request command, but I believe it did and you "saw" any open position(s) by virtue of a pop-up screen. Might have to fire up the old PC and check it - since my mind is getting feeble!

It did not deal with formations and is, IMHO, where the rub comes in... I am certain. When the gun is destroyed, then you would have to leave or just die in place.
I think there was a Navigator/Observer position also, which was worthless, but let you be in the plane just for the fun of it.
EDIT: As an aside, the pilot was not able to be a gunner. <period> It did not take 4 gunners to protect you, BUT the more the better

. Tail and Ball were usually enough for most attacks but in all reality, even a Death Star could be downed fully gunned. (Pilot Killed)