At 160 ppm its still going to be fairly costly to produce pure water but as you say needs must.
It works like this, if you only use one DI bottle then you will more than likely change the resin when it hits
about 10ppm some will let go higher than this but lets use 10ppm as an example.
So you will be throwing away resin that is still filtering the water from 160ppm down to 10ppm.
If you add another bottle then you don't need to throw this resin away until its well spent, even if its taking the water
down to 80ppm then that's still half before it goes through the second bottle. (hope this makes sense)
It means you get more use from your resin so a double DI more than covers its cost.