This is an exercise you'd go through to make your RODI produce water FASTER. Contrary to what you may hear elsewhere, this is NOT something to do if your goal is to reduce the proportion of waste water produced. More on that later if anyone is interested.

You can plumb two membranes in parallel or in series.

PARALLEL
You'd split the feedwater line after the last prefilter (usually a carbon block) and feed both membranes. Each membrane has its own flow restrictor.

SERIES
You'd feed the second membrane with the waste water from the first membrane. A single flow restrictor is placed in the waste water line leaving the second (downstream) membrane.

Russ