One problem you'll have is that if you have any electrical outlets or switches on the walls to be double tiled, the switches and outlets under the face plates will have to be reset and extended to the new double-tiled surface. This can be quite an expense, at least here in the U.S. since, while the job isn't terribly difficult, most "trades" (plumbers, electricians, tilers, etc) won't do work on those items that are not their specialty for fear of violating some code or other. You may wish to check into that...