I am hooking up water lines to a shower that I am remodeling.
Prior to the remodel:
there is a 3/4" water line (in the crawl space) extending up to the area near the tub/shower, it then downsize to a 1/2 inch pipe that goes up into the bathroom and splits into one pipe for the shower and one pipe for the bathtub.
I am planning on making a new shower that will have 2 shower heads, each with their own mixer and on/off valve.
I am also planning on using CPVC.
I have the option of staing with the 1/2 inch that is already running up to the shower and just put another "Y" in it to bring water to the new shower head (with its own on/off valve and mixer). I'm not sure if I there will be enough flow through this system to run both showers at the same time. The water will flow to right up near the shower in 3/4", then down-size to 1/2" copper, then down-size to 1/2"CPVC for the two showers.
The other way that this could be done would be to make the whole system 3/4" by going with 3/4" CPVC from the 3/4" copper in the crawl-space. But this would be a bit more work.
Sorry my explanation is so long.
By the time the water passes through the ports on the valve it will be reduced to at least 3/8", no matter what size pipe you have to the valve, 3/4" or 1/2" it means nothing, run 1/2" the the heads.
Plumber for 20+years