You need at least two handles to do what you want, and maybe a third if you want to divert to a second showerhead. You might want to check out Delta's R10000 rough-in valve body, then you have a choice of two different trim/cartidge types that would work. There are three different ones that work in that rough-in, but the first is a single handle that requires going through cold. Look at the 1700 series. The big-box stores sell these as full kits, but a plumbing supply sells the rough-in and trim of your choice as separate pieces - much more variety.
The middle of the three uses one handle (often a lever) for temperature control that you can set and leave, then the second one only adjusts the flow from off/on. Then, you could also choose a thermostatically controlled valve that works similarly - set the temp with one knob/lever and adjust the volume with the second.
I put one of the Delta units in at my mother's house. I have a thermostatically controlled Grohe valve in my home that I like.
Personally, I find the hand-held on the shower bar fine as my only showerhead.