We run two 40x40
Run waste into second membrane, so there's only one waste to adjust, membrane in line keeps pressure more regulated
At 1000 litres a day I personally wouldn't bother with 2 membrains one membrane with booster will produce about 250-350 litres an hour
If possible Just have more tanks for storage so it's starting with 2000 empty 1000 even if you come back 3 hours later still enough water
We supply 13 -14 vans we run 2 40x40 as you can squeeze the ratio of waste and gain a few extra litres a minute we make about 450 an hour
Waste ratio is personal I keep 70 % but are happy to replace membrane once a year arriving at r/o at 003-005
50/50 is good for longevity, possibly higher rejection, but there's a sweet spot to aim for, you'll be able to here it whistle it will be where water waste to tds is at it best ratio to kept pure,so so in other words you'll not get a lower tds but can reduce the waste,
That's what I aim for also balancing the cost of water rejection with resin cost is also something to consider
We use sometimes 40,000 litres a week so these so these small calculations down the line incurs cost
Hope that makes sense and is helpful