I just got to port and finally had the option of installing 5.25 Stern + Bow deck guns. Finding that much more useful (in theory) I installed the upgrade and proceeded to the crew tab to get new recruits for my second gun.
Needless to say, there was no new slot for the rear crew. The front gun still had its crew (which was where I originally had a 4.50cal before installing the 2 5.25cals).
So my problem is even after doing a mission there is no button to go to the rear gun, nor a spot to put crew to man the rear gun (though it does seem to fire in combat on its own?)
If I remember right that is a problem with SH4 but once you get a newer sub (which should happen soon) the problem is fixed. I think.
What sub is an upgrade from the Balao? I'm in the last 6 months of the war and I haven't been offered a new sub....Also, I too have dual deck cannons and only 1 crew.