Depends...are you going to use a divertor in the tub filler spout, or a separate valve? If you are going to use a diverter valve, then one volume control would be fine. Actually, cap one end and run it to the volume control, then either t off to both the tub filler (with divertor) and the shower head, or to a separate divertor. There should be some instructions in the box...what do they say? Grohe makes a bunch of rough-in valves (1/2"-3/4", with varying flow rates). Which one do you have (model number)?