I'm not entirely sure about this, since it's a fuzzy memory dating back to my primary school years of sneaking in the storage room and reading my fathers collection of WWII aviation and history books (like the Salamander encyclopedia of WWII aircraft, i still have that and it's great, tattered hardback covers and all) when i should be studying
In any case, i might have read in some book among those that the rear MG was probably fired by the pilot but it wasn't aimed, so it was mostly in an effort to scare a pursuing fighter that was attacking from dead six rather than score hits. Since the top turret gunner's field of fire probably doesn't extent that low, maybe he would tell the pilot over the intercom when to shoot?
I might be entirely wrong, after all the luftwaffe was famous for weird gun placement on their twins and the Ar234 Blitz bomber also had rear-firing armament that the pilot could not only fire, but also aim through a periscope sight. I'm sure others will shed more light on this.