Perhaps they're just too close and overlap to the point it's uncomfortable to make the choice? Swap the Mamiya 645 stuff for a good 35 rangefinder and some kit, and add to the RB 67 a little as well. That way you have two "different" systems. I've shot quite a bit with an RB 67, never with a 645, but have handled one a bit,,, 645 just didn't seem like a good "point and shoot" format for me. You've got the studio type of photography covered nicely with the RB. (if you're a submini format guy ;-) )
Or,,, if the 645 does good for your point and shoots as well doing studio duty,,, dump the RB and get a 4x5 field camera. I just can't see a lot of difference between the RB and the 645. (Ok,,, religious war to start here I'll bet).
Questions begging to be answered. You need more of a point and shoot for day to day stuff? Easier to handle did come up about the RB. Do you really need bigger film than 645? Grain bother you? Need the available light stuff with asa 3200 and so on? Is the RB format large enough? Does the 645 really fill the 35mm slot for you? Or do you even need that format? Is 6x7 really an advantage over 645? Have you found it limiting and too small at times? I assume you really don't need any real camera movements.
For me,,, I just have a major problem with grain. Why I've still got my Nikon's is just a matter of my rules, see below. When I was using my buddies RB, it was as big a fuss for me as my Linholf Technika. I just never really bonded with it, and the format didn't do anything for me that 2 1/4 square didn't. I really miss the C33. (didn't sell any of these,,,all loaners from the same buddy that just swaps equipment like some folks do wives and cars.)
So much for my rational answer. "MY" answer? I never sell cameras, guns, fishing gear, Ham radio stuff, or RC planes. (or pets, or wives for that matter). Two does not count as a collection of cameras, I'm not at all sure it's even proper to start counting when you have fingers on one hand left over. Keep your two systems, add a good rangefinder and a 5x7 or 4x5 if you must, and be well!!!
What do you think the chances are of any of the folks on this thread actually doing the one camera, one lens, one film, one year class is? So much equipment,,, so little time ;-)