Given that anyone can literally switch between the two programs at will, if what you say is true, then just switch your program to RC when you have an RC stay. Then switch it back to Marriott when you have a stay there. I know of no restrictions on switching back and forth so it shouldn't be a problem. (Though I don't remember if you can do it online or if you have to call MR and have them do it.)

This way, you will be an RC Plat when you are staying at an RC hotel. You will be a Marriott Plat when you are staying at a Marriott hotel.