I don’t think this is true. I think they could do 1 carbon, and 2 di, they would just need to fix the order the water flows through. Also, I have read if you are going to do a cation and anion separately, you would still need to do a mix carriage, is this true?In order to have 2 di resin canisters you will need a 6 stage system. The one pictured is a 5 stage with only 1 being a mixed bed of DI resin. Imo if you go this route I would separate out the di resin and just buy bulk bags of cation and anion this way you can just replace the anion as it will be used up at a much faster rate than the cation. With mixed resin you are pretty much throwing away good cation every time you change it out.