Solve the interchangeable back issue and keep your love for the Yashica by buying a second Yashica and loading it with different film. Your accessories will work and your workflow will be unchanged. Of course, that combo might be a little bulkier than an MF SLR with two backs, but probably not by much. I don't have a 6x6 SLR to compare with, but I'd guess 2 Yashicas would be smaller than a Mamiya Pro with 'normal' objective and two backs and slightly larger than a Mamiya 645 Super/Pro with normal objective and 2 backs. I'd bet the 2 Yashicas would be lighter than either two SLR systems I mentioned. Good luck!