its a American tank. and American driver usually on the right side. So the 2nd gun has to be there. There ia a 37mm gun on top of the M3, which also considered a main gun in 1940.
If enemy appears from left, the commander on top of the tank will use the 37mm gun on top first. He scan also turn his tank left to duel with the enemy head-on.
sorry, my computer only type english.