I had an extra di canister and thought about just adding it as a second di stage in my rodi setup.
I checked the tds before water enters the di canister, and it was already just 4, so I figured it would not do much good adding another di stage.
Then I thought about using it on the waste water line and using the water as well. The tds of waste water was about 170, which is higher than the tds of the tap water at 160. But the di stage on the waste water line was still able to get tds down to 0. but for sure the resin will not last. So in the end, I ended up just storing the extra di canister I have.
so imho, the second di stage is not necessary.