Originally Posted by oh'mike
Any that I have seen require 3/4 " dedicated lines.Check the manufacturers spec. sheets --they know.
You can't take a mixer that is designed for one head and expect it to produce enough flow for two.
Good luck--have fun--Mike---
I recently installed a Moen 275 Spa set which has a thermostatic valve (mixer), showerhead, body jets, and a handheld. However, you can buy the mixing valve separately. Each function (showerhead, body jets, and handheld) has its own shutoff valve. I don't see why this couldn't feed two showerheads instead. The supply lines to the mixing valve are 3/4". 1/2" can easily come off of the valve to each showerhead. I placed the handheld at the opposite end of the shower (4'x6') for the little lady to use so I wouldn't have to get out from under the big showerhead and bodyjets :-). I ran one 1/2" line under the floor to the handheld from the 3/4 output of the mixer.