| We did that exact same thing in our bathroom. I have the water supply going into the Grohe thermostatic controller. Ours is laid out so that each of the three things, the tub spout, the hand shower and the overhead shower, has their own on/off valve. This way you can use any of the 3 or all of the three. Our tankless water heater can only supply 7 GPM so you can't effectively use the the tub spout at the same time as the shower head. Our master bathroom shower is similar, but has a hand held, rain head, and 4 wall jets. I can use all three at the same time, but the pressure goes down a little. If that doesn't make sense to you I can post a picture. I absolutely love this system. My wife set it to 90 degrees, I set it to 100 degrees. No guess work, just set the knob to the temp you want. With the tankless water heater I can stay in the shower as long as I want. One thing you need to be aware of with tankless in case you aren't, is that while you will never run out of hot water, you ARE limited to how many people or appliances can use it at the same time. For instance, I can't take a shower while someone else fills the bath tub. The bath tub uses nearly 100% of the water heaters GPM capacity. That will result in my shower being diminished to a trickle. ~Jay | 
