There are 3 ways to run dual membranes,
The first runs them in series where the second membrane processes product water from the first membrane and has 2 brine lines to a drain. This is wasteful and not necessary, but allows you to get super long life from your resins. Since you are getting 0 tds out of the second membrane this is likely how yours in plumbed.
The second way to run dual membranes is to process the waste from the first membrane with the second. This is typically used when water conservation is important, it will always raise the overall tds and consume more of your resin life, but greatly reduces the brine to the drain.
The 3rd method just runs 2 membranes in parallel. You T off the booster pump to feed both membranes, and recombine the product water before the resins. You will have 2 brine lines, and if you use a float valve on your storage container you will need a shut off valve for each brine line as they work independently. I use mine this way and it literally is like having 2 RO systems, it makes water twice as fast. I can make 5 gallons in about 23 minutes @ 65 psi.
You really need to monitor pre and post membrane tds, as well as the tds after each resin stage. In addition it is a good idea to monitor the water pressures of your tap, post mechanical /chemical filters and post booster pump. You can diagnose your system faults by knowing these values.
And most importantly, to prolong the life of your resins it is imperative to bypass your resins to the drain until the tds has stabilized after you first start making water. The tds inside the membrane will equalize and could take up to 10 minutes of use before your target tds is achieved. If you monitor it at all of the points you'll see what I'm talking about.
Make sure you have the correct restrictors in the brine lines. Depending on the TDS of your raw water, you should run 3:1 to 4:1 for the longest membrane life. I don't know off the top of my head which restrictor you need but that info is easily obtainable on the internet.
Degassing well water may or may not be necessary, a C02 test is necessary to determine that. If your pH rises by just letting the water sit overnight there's a good chance it contains excessive C02. I'm transitioning to well water and will probably have to do the same thing.
