You also need a wall supply. Depending on the handshower you choose, you may also need a separate check valve. I was told in a showroom, that Moen puts their check valves in their handheld shower, so the wall supply doesn't have one. But, you can buy one. Hansgrohe, however, doesn't put one in their handshower, and you can buy a wall supply with or without the check valve (I believe), so you need to make sure you buy the right one. It is okay to have more than one check valve, but you want to make sure you have one.

Nancy, I have the Grohe Atrio trim, too, with a handheld, although not on a bar. It's a different configuration than your's, but I'm posting a picture for you because I think you can center both. In my picture, the part on the 7.5" plate is the temp. control, that would be your pressure balance valve. You could easily put a 24" bar above that (the tiles you see are 13 x 13). Hope this helps. (P.S. We're using a Waterpik shower head from Costco which is fabulous -- took off the Grohe Movario).

Glad the threads you read were of help. Easiest, and often times the most user-friendly, is two supply valves, one for each head. With two bathers, each can select their own water temp. In general, that can be achieved most economically by using two two pressure balance valves at an economical price. Diverter valves and their trim can often be as expensive as a supply valve and its trim. Obviously you can price things out through the manufacturer of your choice: -Two individual supply valves and their trim. -A single supply valve and its trim plus a diverter and its trim. When you get into diverters, then you have all sorts of choices...the diverter can supply one head OR the other, but NOT BOTH simultaneously...OR it can supply one head OR the other, as well as BOTH simultaneously. As you wrote, with a PB valve you may not get temperature control at any volume. Volume control at any water temp can be of value, it depends on how you use your shower. To do that, you're right, it can be accomplished with a thermostatic valve. To be fair, there are some PB valve that offer that as well, you'd have to shop around to find one. But for the most part, PB valves at low flow will only give you cold water. My own shower, we have two PB valves that supply two individual heads. I do like the ability to have different water temps out of the two different heads. You're certainly on the right track. It's simply a matter now of balancing features versus budget, etc.

My designer was here today helping me with some staging and I asked about my Speakman shower heads. She said that Speakman manufactured shower heads for some brands but they weren't advertised as such but just sold as part of the brand. For example, my shower fixtures are by Phylrich which uses the Speakman shower head under its brand. Evidently Speakman has particular design/construction elements which are recognizable that a professional recognizes a "Speakman" shower head even when it is not branded specifically but is being sold as part of the plumbing "package" by other brands.