With a prop on each end of the shaft, the blades will need to be pitched the same way when fitted. With the bosses facing outward, this does mean one left and one right hand prop. As a previous poster mentioned, blades on most model props are not symmetrical. Matching the performance of the pair might be a problem. This will apply whether they are on a single fixed shaft or a coaxial pair, when they can be counter-rotating, but will both then need to be the same hand.