First off, you are missing a couple of "Y"'s and another "Valve" on the output of the first membrane.
You need to "Y" the output of the first membrane to go to the input of the second membrane, or waste water (this is where your second "Y" and "Valve" will go).
Doing the above allows you to flush the first membrane, otherwise, with your current connections, you will never do that.
I run my booster pump before the second pre-filter/charcoal canister/coconut charcoal canister (Chloramine)/membrane. Works beautiful. Here is my setup.