Assuming you are going to tile this new shower, I suggest you look over at www.johnbridge.com They have numerous experts dedicated to building showers and tiling in general. If you have copper pipes, two showerheads is probably not a problem. Most showerheads sold in the USA are restricted to 2.5 gallons/minute. Nearly any shower control valve can pass that amount. Now, having a large enough hot water heater to satisfy some people is another thing! More than two, may require 3/4" supply and probably a really big hot water tank (plus a valve with a large flow capacity). You can install either a cap on the existing pipes while you get things ready to finish up, or use a compression shutoff valve. If you will have access to the plumbing after the finished walls are up, you could put in permanent shutoff valves. My unprofessional opinion.