Hi! I see your idea and I think its great your using the waste water for something.
In that particular RO setup, or any for that matter I don't believe you can make it more efficient by adding a second membrane (unfortunately) in the way you describe.
The biggest thing that comes to mind is pressure, the waste water doesn't have enough pressure to drive it through a membrane. There are more efficient ones, up to 1.5:1 (or even 1:1) units but those are designed to maintain high pressure though the single or double membrane.
Also, the DI is used to reduce your TDI (total dissolved solids) to essentially 0. The water coming out of the membrane might be close to 0 depending on your source water but the DI membranes finish off whatever small charged ions are left in the water. So yeah, even with a super efficient RO and multiple membranes small charged ions still make it through. So if 0 TDS is the goal then DI is still needed.
Here is an example an efficient "water saver" unit for some more info: https://www.bulkreefsupply.com/4-stage-value-150-gpd-water-saver-ro-di-system-bulk-reef-supply.html