Kugai, you're thinking along the lines of the two-seat SA-25A Badger heavy fighter seen in the episode "A Voice In The Wilderness."
The rear guns on the SA-23E Aurora supposedly were intended for rear coverage, although I doubt most pilots would want an opponent to place themselves into a position where either could take the kill.
Disregarding the rear guns, then, surely someone can explain why only the lower pair of guns were used instead of all four together.