I'm not sure why you need the second cable release at all for the RB67. I recently got a ProSD, so maybe this doesn't apply to all models, but when in mirror-uo mode I just flip the mirror with the button on the body using my fingers. Since no exposure is made at this time, I don't have to worry about camera shake. I wait a second or two, then use the cable release to fire the shutter on the lens.
If I'm not using the mirror up mode, I switch the cable release to the body.