understanding multiple ro membrane use

[lastlight]

So I've watched the brs video and it seems you run the waste line from membrane 1 into the input for membrane 2. The way I understand it (assuming a 4:1 waste to pure ratio) you reduce the volume of water by 20% or so (4 gallons waste from 5 inputted) so the tds in the water leaving membrane 1 should be 20% higher?

If I follow this right the outputted water from membrane 2 is going to be slightly higher tds. The end result of this dual membrane conversion would be less waste produced overall and a slight increase in the amount of di resin consumed?

Anyone here do this upgrade? Pros or cons? I'm aware of the pressure requirements...

[Dyspnea]

TDS numbers will be generally unaffected. The multiple membrane use appears to be purely provide a reduction in waste water.