I got the 100 as my membranes are in tandem( ONE MEMBRANE LEADS TO THE NEXT ) I think most use them this way.If you have 2 membranes producing water separately then you will need the 200 divide the pump water tween the membranes. Good booster pumps and a very good price. Take a look at other products John Hewett has good stuff and great value and been a windy himself knows the industry.Using these pumps it increased my pure water production well.