My water goes through DI before the pump, I used to have twin 11 litre DI's in series.
But after a while of being lazy I found that the first DI had gone higher than my Tap TDS and was therefore making the 2nd DI last less than it would on it own. I went back to single DI, I keep a tally of how much water I use and know roughly when it's going to be time to change resin, or to check the TDS more regularly.
If you search the old posts on here for twin DI there's bound to be more info
Dave