So most of this is fairly correct but there is a lot of grey area.
Membranes are commonly spec'd at 15% recovery which is approaching 6 to 1 waste to product ratio. I have never seen a system set up that way. A lot of references will say 4:1 but I also haven't seen many higher flow 75-100 GPD residential systems set up 4:1 with 800-1,000ml/min flow restrictors. Fact is, it isn't a perfect science and completely dependent on not just your TDS but also whats in it. For instance, someone's 400 TDS water may perform better at 4:1 but someone with 100 TDS source water may very well get away with 2:1. It also depends on what is in the water, the 400 TDS water might last longer than the 100 if it has been softened and most of the hardness has been replaced with sodium which doesn't scale the same way.
The ratio on your system is pretty easy to figure out. a 75 gallon per day membrane emits ~200 ml a min so 2:1 would be 400 ml of waste/brine a minute. 3:1 would be 600 and 4:1 800. To identify how much waste your system produces you could measure how much waste comes out in a minute or just look at the number on the side of the flow restrictor cylinder which represents ml/min.
Today it is much more common to see 2:1 or 3:1 simply because most people don't want to waste tons of water and today's membranes last a very long time in even the toughest environments. Unless you know you have terrible water I personally suggest running a fairly low ratio and only changing if you find the membrane doesn't last as long as you like or having rejection issues. Almost always makes sense to start low and work up.
There is also pressure to consider. On many residential systems 2:1 or 3:1 might perform better on higher flow residential 75/100 GPD membranes because the lower flow ratios are able to maintain high pressures and pressure is one of the most important performance specs. High pressure almost always = higher flow rates and lower TDS product water.
As to the two membranes in the water saver configuration, there is actually the "more" flush than installed in a normal configuration. Its hard to grasp but this is how it works. Let's say you have 100 TDS source water, 75GPD membrane and want a conservative 2:1 ratio. That would require a 400ml/min flow resistor. Then at a later date you add a water saver dual membrane kit where one feeds off the other. this is what's going to happen in each membrane.
Membrane 1 has 100 TDS feeding it and likely has 98-99% rejection with 1-2 TDS product water. This membrane has no flow restrictor on it and relies on the pressure from the second membrane. The flush ends up being all the water feeding and leaving the second membrane which is 200ml/min product + 400ml/min waste. So membrane 1 has 200ml product and a total of 600ml of waste or flush and an increased ratio of 3:1. So even better than you had before.
Membrane 2 is fed the c0ncentrate /brine/waste of membrane 1 and rather than 100 TDS it is likely now ~130 TDS however with a rejection rate of 98-99% it is going to be the same 1-2 TDS and once the two products are mixed together there is almost no real difference. In this case, the product to waste will be the intended ratio of 2:1 with the 400ml flow resistor and 200ml from the 75 GPD membrane.
Math is slightly different as you scale this up but even if you ran at 5:1 in the second membrane, the first membrane will always have even more flush.
