Dual membranes ARE NOT water savers.
If you desire faster output a dual membrane system will increase the GPD provided you have a minimum of 65 psi at the tap to operate it efficiently.
Those who claim dual membranes are water savers are kidding themselves. Think about it for a minute, you are taking the waste or concentrated brine out of the first membrane and sending it to the second. In a normal 4:1 waste ratio system that brine is already 20% to 25% higher TDS than your tap was to begin with so the second membrane already has more work to do than the first. Now you reduce the waste even further and the brine increase even more, what is going to happen? The membranes fail due to fouling or plugging and you spend money to replace them much more often than if you had just left things as they should be.
If you are considering reducing the waste ratio below the industry standard 4:1 you first need to know your tap water calcium carbonate hardness and your tap TDS. If you have hard water and normal (250 is about the national average TDS) to higher than normal TDS then forget about it. If you have softened water or a water softener then you may be able to reduce it some, still depending on the softened TDS. If you are blessed with both soft water and low TDS as a few places in the US have then you might be able to reduce it as low as 2:1 and still get reasonable membrane and DI resin lifespan.
Unless you want to make more water faster I would stick with a single 75 GPD membrane and fine tune the waste ratio using a capillary tube flow restrictor you yourself trim exactly for your own unique water conditions. You need to know what those conditions are though including water pressure, water temperature, TDS and hardness.
