Hi Juha,
You know, war = no rules. Theoretically Kingfisher was armed with flexible gun at radioman position. Theoretically. Try to find its typical AN M2 machine gun behind the pilot in the rear-based Kingfishers when the life rafts replaced the guns in radiomen's cockpits.
When it comes to the Russian P-39s -- theoretically perhaps you are right. The Russian manual of 1944 I mentioned shows P-39 cutaway with complete set of two guns per wing. Heaven knows what the units did in the field however...
Regards
E.