The George was one of the earliest planes added to the game. I seem to remember when it was added, Pyro stated that they wanted to add a Japanese plane that would be competitive with the planeset at the time (Spit, Pony, 109G10). He also stated that every other online sim at the time had the Ki84 in it and none had the George. As far as the reason for having the 2 vs the 1, I would guess the performance was the issue at the time it was added and they may have had better information on it at the time as well. I seem to recall the the 1-J had some serious issues due to its being adapted from the seaplane version rather quickly (issues like the weak landing gear and some stability issues from the midwing were quite common in the earlier version from what I understand). It wasn't until the N1K2-J that the airframe became a truly effective fighter.
Who knows, its a fun plane to defend in.
Also, if you get a chance, pick up "Genda's Blade" by Henry Sakaida. Its about the 343 Kokutai. This was sort of the equivalent of the more well known JV 44 in the Luftwaffe. It had the highest number of aces and veterans in the IJN at the end of the war and they flew the N1K2-J. Its a fantastic book with a wealth of detail and research.