You CAN use the 420, BUT, you have to connect the discharge from the hot and cold valves together into a "T" first. THEN from the T you run a pipe into the "single" inlet to the diverter. From the diverter's two outlets you run to the spout and shower head. It can get somewhat cumbersome as far as the piping is concerned if you want all three handles in a straight line, and you may have to use an 11" c/c spacing to give enough room for it. You do NOT use the spout with the diverter knob.