The chances of spinning 2 props is unlikely, but I guess possible. With a single, a spare is important, with twins, the other engine becomes your clutch if something goes wrong, just do not over power the boat o0n one, it will not get on plane. If you do plane to carry props, aluminum is cheaper and you will noty be worried about economy since it is just an emergency prop, I would not buy SS, not worth the investment. As for which one...not sure really since you really can only use it on one engine and you only have a 50% chance that that is the one you might damage so carrying just 1 may not help. My advice....get a tow boat plan somewhere, they typically tow up to 20miles under their plans, and if your going over 20miles you should have a buddy boat with you anyways or know someone else out there incase something goes wrong.