I'm working with a home builder to have two shower heads in a master bath shower. The builder proposes have two shower valves, one per head. Is this recommended/required or is the builder just trying to charge me more for another part?
If you don't want both shower heads to be on at the same time everytime you take a shower, then yes you need 2 controls. If you don't care if both head are on, then one control will be fine.
I have two heads in the master, I don't remember any charge but I guess there was. You just add inexpensive shut-offs on each head , works fine. BUT , in the US , regulations require that these shut-off valves leak, so if you have a head turned off ,it will still drip. We usually have both on at the same time. It did take a couple trips to the "big box" store and one trip to the plumbing store to find out that these shut-off are required to leak ( in US).