Shower valves are complicated systems, but basically it boils down to a series of simple questions that build on each other: (1) how many "water sources/shower heads" do you want and where are these to be located? (California now only allows one shower head, so plan accordingly in California) and (2) do you want to operate these simultaneously? Two additional questions to ask if there are two or more "water sources" in the shower is: (1) will two persons use this shower together or on a regular basis; and (2) do they like their showers at the same temperature (I find that men tend to prefer cooler water temperatures for showering.

Shower valves can control three things: (1) water on/off and (2) temperature -- either by mixing hot-and-cold (pressure balanced) or by mixing hot-and-cold to a preset desired temperature (thermostatic) -- I'm over-simplifying, but you get my drift, and (3) which "water source" the water is directed to....and this is accomplished via diverters or volume controllers that can be integrated or separate from the valve. (I told you this gets complicated! -- now you see why you get lots of different answers when you ask the question!)

Since your goal is to have two "water sources" that you operate simultaneously -- the rainhead and the hand-held unit, I recommend you install your thermostatic valve and two volume controllers. You will preset your temperature at the valve and probably never touch it again unless you want to change temperature. Your volume controllers will be basically "on-off" devices that will allow you to turn on one or both "water sources" simultaneously. With this configuration, you are able to locate the volume controller for the most-often used water source (the hand-held I would presume) closest to the shower door so that you can start the water without getting wet. Once you're in the shower, then you can turn on the rainhead.

This may be more information than you were looking for, but I hope it empowers you to make good decisions and get what you want in your remodeled shower.