Subfloor question for tiling a bathroom floor with heat mat under the tiles

Ok I will most likely go with ditra, I was hoping I could tile directly over the 1" plywood to keep costs down the contractor also keeps assuring me that it is solid. Any issues down the road and I will be the one to call tough, so for future reference there must be 2 layers of plywood even over ship lap, I may also look into some other membranes that are more cost effective maybe a peel and stick membrane such as easymatt or mapegard

That's not what I said! If the layer you have has a 'D' face, then I'd want one on top that didn't! You never want a void in a floor you are going to tile. The fact that it is strong overall has nothing to do with a point load over a void. A small deflection there can crack a tile. Natural stone wants two layers, but normally, one over dimensional lumber is fine for either (if it meets industry standards, that is).