We're using a pair of 5603 single-bike triple cranks on our tandem. We have 26-42 in the inner and middle drive rings and a CDX belt drive in the outer ring position for the timing drive. The 5603 cranks have slightly more clearance behind the crank-arm for the belt ring, which is why I got those instead of the newer 5703, but if you're running a chain instead of a belt then this difference wouldn't matter. As mentioned above, 6703 uses an odd mounting and BCD for the inner ring, so I would avoid it. Stages cranks are available for the 5703 cranks, one of their cheapest options, and the left-hand crank mounting interface is the same on many Shimano cranks (at least it is for 66xx, 56xx, 57xx, probably 67xx and others), so it would be pretty universal and easy to switch between bikes; only the styling and finishing differs between the models - I've been considering getting one of their LH cranks for my single bike.