You have a diverter installed in the wall with shower plumbing. You will have one control for the shower and temp and you will have a another control to shift water flow to the handheld. I had what you described after our master bath remodel. It was ok. But on our current build, I'm doing the inwall divereter. It really isn't that more expensive if you are building.(Basically the cost of the diverter and trim piece)

It would look something like this

jimnyo what you had before was an On-Off Valve, not a diverter.
They still make them.

If you want to run both at the same time, get this setup again.

thank davidro1. but i was under the impression that some rough-in valves like the thermobalance III (and maybe an alsons one? i vaguely recall someone mentioning that) have an in-wall diverter that lets you use both a fixed head and handheld at the same time? if so, other than aesthetics, is there any reason to use the in-wall other than the external one? or is it personal preference, like what bh401 was saying?

"... under the impression that some rough-in valves like the thermobalance III (and maybe an alsons one..."
please post again with a verb in that sentence.

"...vaguely recall someone mentioning that) have an in-wall diverter that lets you use both a fixed head and handheld at the same time..."
Please talk to a real store person and get clear what you

Have you considered simply having ONLY the handheld? If you get a nice one, and make sure the slide bar is adjustable back/forth and side/side, it's more than good enough as a regular shoiwerhead. This is what we did in my teen daughter's bath. The slide bar is mounted high snough that the showerhead (a good Grohe Relexa one) is high enough even for my 6'6" brother to be comfortable under as a regular shower, but it can also be slid lower for kids, and taken off the mount to wash the shower out. In this situation you can use a normal tub/shower type valve, simply running the top, shower supply to the handshower ell.

Here is a link that might be useful: bathroom renovation for teen girl

We used a diverter. Much less clutter. We also have a handheld on the tub (no picture to show) and am using the same diverter system for the tub.
{{gwi:1402756}}From Zigman Master Bath
{{gwi:1402757}}From Zigman Master Bath

sorry, davidro1, that's what i get for posting at 1.30 in the morning, lol. lemme try again:

"but i was under the impression that some rough-in valves like the thermobalance III (and maybe an alsons one...i vaguely recall someone mentioning an alsons valve that also lets you run both heads at the same time) have an in-wall diverter that lets you use both a fixed head and handheld at the same time? if so, other than aesthetics, is there any reason to use the in-wall other than the external one? or is it personal preference, like what bh401 was saying?"

that said, i was at a fixture shop today and they said 1) the little thingie IS called a diverter, more specifically, a "shower arm diverter" and 2) the hansgrohe thermobalance III does run both heads simultaneously. but i was hoping someone could tell me if there's another company that makes something similar to the thermobalance III.

staceyneil, thanks, but yes, we need both heads. when i shower w/my little kids, we need the extra showerhead. the handheld on a bar would work nicely. thanks!