If you take the waste line from one mebrane and run it thorugh a second membrane (in-line)you minimize waste water, it is a sure fire way to maximize water production and minimize waste but the problem is that you need a decent amount of water pressure to achieve this so the unit needs to have a booster pump.
From my research, and my conversation with Jim from "The Filter Guyz", the best membranes to do is are the DOWN 100gal per day. Many companies sell in-line kits to upgrade your unit.
Running paralel only increases production, but doesn't affect waste loss, you'd also need a booster pump in order to push water through the two membranes.
Hope this helps!
Rod