When you've added even more weight in the form of armament, armament mount, ammo drums/feeds and armour protection. Having not yet satisfactorily explained where the bulky WWII radio equipment and, in the case of the P-51, the fuselage fuel tank are going to go, I'd say we've got so much weight aft of the CoG it's going to need an interstate to get it off the ground and fly like a wounded turkey when it does.
Weight and weight distribution are factors of primary concern to single-engined, single-seater fighters. Just about everybody adopted the finger-four during WWII for good reason, it was the best method of catching someone sneaking up on your formation before he could do any damage.
There would be zero benefit to saddling this type of aircraft with a second crewman, and a host of reasons why it wasn't a good idea.