I do 2 mixed beds, when time to change out the first, I remove the first, move the second to the first position, and add a fresh second. This is easy, can be scheduled, and is 1 fewer canister to deal with. 0 TDS 100% of the time practically, and also sound theoretically. I have very low TDS leaving my membranes, so I only rarely have to change out a resin cartridge at all.
The main argument for the separate cation and anion beds is so that they can be changed out only as needed, thereby reducing the waste of changing out a mixed bed just because 1 of the 2 components is used up but the other has life left. This makes more sense if your water quality or membrane efficiency are particularly bad, but I think it seems like more work and more prone to mistakes. Our water quality in the Bay Area is pretty good. and it is straightforward to get high quality membranes.