The system is a couple months old.
How do you turn it off between uses?
If the water runs out of the membrane canister between uses it can go bad really quick.
Curious that have a flush valve? If not should get one easy to install.
What is the pressure into the second membrane?
Your rejection rate there is currently 90%..
That is poor but it could be due to the lower pressure.. (or just a bad/crappy membrane)
And again are you waiting some time after starting the flow of water to measure TDS output?
.Just seems like the second membrane is one that nets around 90-95% rejection rate at the provided pressure/feed temperature..
If that is outside of its recommended specs then its time to replace it..