You won't have separate hot and cold "thingys" anymore; you'll have one valve which handles water volume and temperature. There are two types of values: pressure-balanced (cheaper) or thermostatically-balanced (more expensive). The valve is the unseen piece of hardware in the wall plumbing; the part you see, with the level for adjusting the water, is called a trim plate.

If you want to have two outputs -- a handheld showerhead and a rainshower head, there are additional options. You can get single trim plate which allows you to switch between two showerheads; or you can have a separate diverter valve to route the water to the desired showerhead. Some diverters give you a choice to send water to one or the other (A-B), while some give you the option to run either or both at the same time (A-B-AB).

A greater primer on all this was written by one of this forum's frequent expert contributors, mongoct, a few posts down in this thread.

-- Eric

P.S. That means you're right and DH is wrong. Have fun with that! ;)

Here is a link that might be useful: FAQ/Answers Bathroom Plumbing for dummies

I'm still lost. DH went to a local plumbing store and they told him we had to have two hot/cold thingys. He is really mad at me for disagreeing.

We want a tub spout, hand held showerhead to be mounted on an arm (no bar) and a rain showerhead from the ceiling.

Can someone please spell out exactly what plumbing we need?

Either the description to the plumbing store is incomplete, or the plumbing store staff misunderstands the question. Either way, you don't need 2 hot/cold thingy.

There are a few ways you can do this, but we'll mention 2 common approaches.

#1
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You need 1 pressure-balancing valve (ie hot/cold thingy)
without tub diverter

You need 1 3-way transfer valve (also called 3-way diverter valve)

The pressure-balancing valve will get 1 hot input and 1 cold input from the supply lines. This valve will output directly to the 3-way diverter.

The 3-way diverter gets its input from the pressure-balancing valve output. Its 3 outputs will be connected to your rain shower, your handheld, and your tub out.

In this configuration, the PB valve mixes the hot and cold, and sends one output to the diverter valve. The diverter valve with 3 inputs A,B,C can usually let you choose between A, AB, B, BC, C, and CA, a total of 6 combinations.

#2
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You need one pressure-balancing valve with tub diverter
You need a 2-way transfer valve (2-way diverter)

If you can find a 2-way, that's great, if not, you can still use a 3-way here, and the plumber will know how to do loopbacks and capoffs. It's all in the installation manual as well.

In this configuration, the PB valve again gets its input from 1 hot supply and 1 cold supply. This PB valve with a tub diverter built-in can produce 2 outputs, one to the tub spout directly, and one to the diverter.

The diverter gets its input from the PB valve. It has 2 outputs, 1 to the handheld, and 1 to the rain shower.

Of course, you will need to buy all the trims and covers, arms, spout, heads, handles.

Hope this helps.

If you want the water to the two showerheads to come out at different temperatures at the same time, you'll need two thermostatic valves, otherwise one valve with the diverters will do.
Hansgrohe makes a thermostatic control valve that includes both the temperature and volume control in one thingy. Other manufacturers require one valve for the thermostatic control and another valve for the volume, which adds up to two thingys.

The water for the two showerheads can be the same temperature.

If it is going to cost five hundred bucks in extra plumbing to have two showerheads, then perhaps we won't have two showerheads.....

We just always wished for another one because we shower together sometimes and the person not standing right in front of the handheld gets cold.

No wonder there's confusion. We're talking about different applications. If you want 2 independent shower systems, of course you need 2 thingys.

With the new description, I would worry more about the size of your incoming lines. If you have 1/2 inch supplies and you want to split to support 2 systems, your water rate at the heads will drop when used simultaneously.

Mongo, thank you. DH listened to me read this and we looked at their website and now we understand each other.

He originally explained it to me that we would have to have to large sized hot/cold trim pieces (both identical) and I knew from looking at pictures that the second one would be smaller.

We like the Pasadena collection from Price Pfister which comes with tub spout with diverter, hot/cold trim and hand held showerhead with arm (metal hose)

Looked at Home Depot online, they don't sell the valves alone. We don't like any of the Symmons trim. Are the valves worth buying with trim and not using the trim? Can we use another brand of trim with the valves?

Home Depot does sell pressure balancing temperature control valves alone by Kohler and Moen.

DH is re-piping from galvanized to copper, lines are 3/4 inch to the bathroom now, just waiting for all these decisions to finish. Tankless water heater.

Here is a link that might be useful: would this work?

No the Moen won't work. It can't feed a tub spigot and it'll only feed one shower head at a time, not two.

Glad you got more specific and said you liked the Price Pfister. Going off of that:

Let's run with that Pasedena set that you said you liked. From your description it has pretty much the basics of what you need; a valve, a handheld and hose, and a tub spigot with diverter.

For argument's sake I'm going to use left/right/up/down in describing the valve ports on the main valve. They may be plumbed differently from what I write, I'm just trying to simplify the description okay?

So, install the pasedena valve in the wall. Hot and cold supply go into the left and right sides of the valve. Out the bottom of the valve is the copper line to the tub spigot. Out the top of the valve is the copper line to the handheld hose and head. Make sense so far? That's how it's normally plumbed, but you want to add a rainhead that you can select as needed.

To feed the additional rainhead you'll need a separate diverter valve and a rainhead. So where do we add the diverter?

The diverter will be installed along the run of copper that comes out the top of the main valve and goes to the handheld head. Pfister has a valve called a "Kenzo" valve that's a 3-way valve. They may have others as well, but that's one I know of. I don't know if it comes in the Pasedena trim line, but that's something you can figure out.

Anyway, the diverter valve gets plumbed between the main valve and the handheld outlet. The copper line coming out of the main valve and running up the handheld first goes into the Kenzo's "INLET" port. The Kenzo has two outlet ports. One goes to the handheld outlet, the other goes to the rainshower head.

So after it's all plumbed how does it work?

Turn on the main valve, set the temp, and water flows out of the tub spigot.

Close the tub spigot diverter and water will flow up to the diverter. If the diverter is turned completely in one direction, say clockwise, water will flow out of one showerhead, say the handheld. If the diverter is turned completely in the opposite direction, water will only flow out of the rainhead. Turn the diverter to the mid-point position and water will flow out of both showerheads.

The Kenzo is known as an A/B/AB diverter. Water can go out of one head, a second head, or both heads simultaneously. Not all diverters do that, some are just A or B.

Hope that helps.

YES, that helps! Thank you so much.

Is this the right diverter?

The Pasadena hand held shower/tub spout combo says it comes with the rough in valve and Single metal lever handle for volume and temperature control. So, all we would need is the rain showerhead and this diverter and trim for it right?

Here is a link that might be useful: diverter valve