When you have dual DI you need to monitor the TDS of each canister or cartridge individually. If you are using the same mixed bed resin in both what is normally done is when you first start seeing signs of TDS (tested with a handheld TDS meter, not color changing resins which are very inaccurate and are usually exhausted and passing TDS before you realize it) in the first DI, you move the second DI into the first position since it should not be showing TDS yet, and place a new cartridge in the second position. You get longer DI life that way and better water quality. Even before DI resin is exhausted, indicated either by TDS or color change, it starts releasing weakly ionoized substances such as nitrates, silicates and phosphates. If the first cartridge is relaeasing this to second it too starts exhausting sooner since it is receiving spurts of TDS, usually higher than the tap water since the resin is releasing what has accumulated over time. By monitoring both and keeping both changed as needed you will be better off.
