Of course you HAVE to keep your 124G. As others have recommended, you like the 124G, then get another. Mark Hama is still around so he could CLA a heavily used one and you would be set for life. One color, one B&W, problem solved. I have a D, but honestly, two 124Gs would be the way to go for me too as I have all the accessories too for the 124G.
If someone made a 6x6 back for the Mamiya 645, it would bring new life to that camera system. I love mine but honestly would rather have 6x6 neg vs. the 6x4.5 neg, but I will live with it. Nice system but sounds like you want to do business in 124G fashion.........get two, don't look back.