its simple really .
Use one di until tds starts to rise , when it does add the second di with fresh resin after the used one .
That way the first vessel gets it down to 5 ish then second one to zero , this will save loads of resin as you would normally of just changed the resin and thrown it away.
When it rises again the second di becomes the first and you refill the other vessel with fresh resin.
Hope that explains it , much easier to show than explain.