With the materials described and the foundation used, as long as water can't collect to the point where it starts being absorbed by the 4x4 rails, you're golden. FWIW, I have a 12 x 16 built exactly as you described by Amish carpenters but it sits on a 6" bed of well-drained gravel that's been tamped and boxed and lined w/filter fabric (not landscape fabric). My expectation is that your shed might outlive you. The only 'concern' which really isn't one, is I would put the 2nd 1/2" floor 90 degrees to the 1st layer. This would strengthen the floor. This prevents having 2 seams in the same 'line', thus, actually stressing the floor.
One extra thing I did was get some double-layered chicken wire and deep-tack it along the base on the 2 exposed 12' sides. This way, feral cats, and other large animals won't be able to make the underneath a home for themselves and their families! In short, have good air flow and water drainage and you'll get your $$$'s worth.