FWIW, I've used MLU on my RB67 with a single cable -- connected to the lens -- without concern. I use the body shutter release to prefire the mirror, wait a few seconds, then fire the lens shutter with the cable. Works fine, and makes me think the dual release cables were just to separate photographers from a few more dollars.
Beyond that, I couldn't say from experience how slow the shutter can be on a tripod without using MLU before it affects image sharpness. Obviously, the longer the lens, the shorter the shutter speed you can get away with, but as
@MattKing pointed out, the RB67 is heavy and isn't trying to fire a focal plane shutter in the micro-instance after the mirror goes up -- even in normal operation, there's a delay of a few tens of milliseconds (don't know precisely, but I can see it by eye, so it has to be at least 10 msec), and if the foam pads on the mirror shutter are good, there shouldn't be much vibration from the mirror firing.
Might be an interesting use of a roll of cheap or expired film, to see just how slow you can go before motion blur starts to be an issue.