You can plumb the membranes in parralel, with each membrane's waste water flowing through its own flow restrictor.
Especially if you have good pressure, You can plumb the membranes in series, with the waste water from the first going to the in port on the second membrane. In this case the two membranes function as a single long membrane, If you use two 75 gpd membranes in this way, you'd want a single flow restrictor for a 150 gpd membrane. Make sense?