Not sure I agree with Dazmond here.
Your first container is now saturated. What you now need to do is to use the second resin until it, too, is saturated.
To do that, empty container one and replace the resin. What was container two you put in as container one and you put the fresh resin in as container two. Do this whenever your output from container two goes above about TDS005, as you can pretty much rely on the first load of resin being saturated at that point (as yours is). The reason I say 5 is that, in my case, the odd impurity does get into my IBC (I cut it to get the transfer pump in) so if I dumped at 001 it might just be a little impurity that got into the tank afterwards.
One important point; if you're running two vessels, you don't save a single penny in resin costs whether you let TDS creep up or if you change the moment it goes above zero (above comment notwithstanding).
Vin