The drain for the shower should handle more showerheads if it is the normal 2" drain...if not, then it needs to be replaced, along with the pipe so it is 2" into the main line. Now, if you don't have a large enough supply line (or a sufficiently large hot water heater), the actual spray from multiple showerheads might not please you in eitherstrength or duration. Tub drains often don't end up in exactly the same place, so tearing up the slab is probably going to happen. There is a limit on how far you can go before you also have to move/add the vent lines. Not knowing where these run, you can't really say if that will be a problem or not. So, without knowing your current layout, it will be hard to say. My unprofessional opinion.