How is 2 layers of ceramic tile?

I would have some questions about how it would work, especially in a bathroom. Will the toilet need to be raised? Will there be enough clearance for the door to open? Will there be a height difference between the bathroom floor and the floor on the other side of the bathroom door?(possibly causing a tripping hazard) Will the tile adhere well to tile or would he put a sheet of "wonder board" or some kind of backing? Can it be completely sealed to prevent water from getting between the layers, and what would happen if that occured?  I would ask the tile setter all of those questions, and ask to see some jobs where a second layer of tile was installed so you can know how it really will look.

Putting tile on top of tile sounds like a "quick fix"...what specifically don't you like about the new tile? Color, pattern, quality of the tile setting? If the problem is the tile setter's fault (like poor workmanship) then you may need to negotiate with him to get him to re-do the job right. If the problem is on your end (like you chose the wrong tile) it might be better to pay to have the first tile job removed and the second set of tile installed directly onto the subfloor. Just my opinion.