You can get two 4.5x6 exposures on one sheet of 6xx (664, 690 etc 3 x 4) Polaroid. I'm not familiar with the Mamiya version of the holder, but the usual way is to pull the first (white tab) sheet out part way - so that the tip of the tab is 2 inches (62 mm) from the slot in the holder. This assumes that the first exposure was made in the right place. Some holders have a wire frame that shows you how far out to pull the tab. No doubt someone with a Mamiya back will add better information.

Best,
Helen