Going from a real world logic stand point the problem is you can't create extra items from nothing.

Put this to carpenter where you produce logs. Currently you need 3 logs to make 1 lumber. Assuming you used all 3 logs to make the 1 lumber (ie the current normal synth) then you would need 6 logs to have enough materials to make 2 lumbers.

Real world logic aside since this is a video game I'd be all for this idea. It was possible to for this to happen in FFXI for items you would end up with more depending on the HQ you got. It was all random through so no way to increase the %.