Real ships did have Chase Guns
From : https://en.wikipedia.org/wiki/Chase_gun
"From 1799 Royal Navy frigates were universally supplied with two bow and two stern chasers, "
I hate artificial limitations. If its physically possible to place a large cannon facing forward and back then if should be allowed.
If someone wants to build an ugly platform on the back of their ship to mount four large cannons (say) then so be it.