In parallel I assume? As opposed to series cause I couldn't see how that'd work.
I don't get it...
Do you need that much water?
How much do you use a day?
Does it save waste?
Why do that rather than put a pump before the RO 150psi they're good for yes?
In series reduces the amount of waste. as you are feeding the waste from the first r/o into the inlet of the second. The downside is that the second membrane has to work much harder and will be the first to fail at your high tds.
2 weeks ago I measured the taps water tds which was 100. The pure tds was 2 before di and the waste tds was 140. 140 isn't to bad to be putting through the second membrane, but at your tds of 350, running them in parrallel is the only way.