Hi, I'm installing a Moentrol 3520 flow valve with a Moen 3375 transfer valve to divert flow to either a fixed shower head or a hand-shower. I'll also have a toe-tester tub spout with a diverter. If I had only a single shower head, I would plumb down from the flow valve to the tub spout, and up from top port on the flow valve to the shower head. But with two shower heads, I apparently need to cap the top port on the flow valve and plumb down from the bottom port to a twin ell and then from the twin ell up to the transfer valve. Why can't i just plumb from the top port on the flow valve? Why the complexity of the twin ell?

Install a three function transfer valve (or 6 function). That will allow all three separate individual (or shared) functions.

agree... get another transfer valve with more outlets

One reason is that the "transfer port" between the bottom opening and the top one is very small which would limit the amount of water to the transfer valve.

I only have two shower heads, so a transfer valve with more than 2 ports would serve no purpose. The use of any transfer valve turns the configuration into something Moen calls a vertical spa and if you have a tub spount, their instructions say to use a twin ell.

But in a single showerhead situation, you use the top port of the flow valve. I don't understand why that won't work when you put in the transfer valve.

moen may know something we don't?

I'm just saying why Moen may give those directions, NOT why it would not work.