I agree that it could be very beneficial for M and J to talk directly, just as YouAreHere suggested. Hopefully that will happen (I have suggested it to J, and both M and J had proposed the idea separately to me a couple days ago; hopefully it will happen sooner than later)
london- I agree, to a point. I think it's along the lines of what LovingRadiance had to say- that as long as another partner doesn't have intentions of trying to drive a wedge between the two in conflict, that sharing details to get support is probably okay. But again, as Marcus noted, I think it could result in a choice between maintaining the level of deep intimacy I have with J, cultivating intimacy with M, or settling for a more distant V in which J and M don't interact as much as they have up to this point.
I'm sure it'll all shake out fine, but it has been pretty confusing for me to figure out how to move forward. I think at this point having J and M talk directly to each other feels like a good step.