Diamond core bits can drill nice clean holes in any tile. If it tile isn't very hard, less expensive tools can cut it.
Typical showerheads are rated at NGT 2.5gpm. Keep in mind that a typical 1/2" supply valve may barely be able to keep up with two heads, which means the performance may not be ideal when adding a second one. It might, but it might not. THen, consider that you'd be draining the water heater twice as fast, and that could be an issue as well. The line to the second head would mean lots more water to drain out, and you'd have a cold water shot for longer before hot arrived, and longer to achieve any change in temperature for the second head.
If you have access behind the walls, say by cutting a hole from the room next to it, you may not need to mess up any tile work. Otherwise, it could be quite expensive and trying to match existing tile work can be problematic unless you have a bunch of spares from the initial install.
Without knowing the ease of access and what actually would need to be done to run the added line, it is impossible to say what it may cost, then, every location has its own local labor costs - say NYC would be much more expensive then east Podunk out in the middle of nowhere.