FWIW, I stay mainly at JW MR, MR, Ren, and RC properties. I have no problem ignoring CY, SH, FFI, and RI threads. I also often use the search feature (heaven forbid) and will search for specific hotels and/or cities I am interested in. Doing this in one forum would be much more convenient IMO especially since the programs are the same. Same rules, same redemption opportunities, same hotels. The only difference are their promos. Not to mention the vast resource of information already available via the search function regarding the aspects and nuances of the MR and RCR programs (same rules remember).
So my vote is for a joint Marriott Rewards & Ritz-Carlton Rewards forum. It is that already with the exception of the name.