Double DI with 0 reading from both DI's

Hi all, is there any point doing double DI if I am getting a reading of 0 out of both DI's?

My first DI gave a reading a while back of 5, so I half filled a second DI with what was left out of a bag resin.

Then a couple of days back I started getting a reading of 4 so I got a new bag of resin, emptied and refilled the first DI tonight and then topped up the second DI with extra resin and its giving a reading now of 0.

So am I better off not doing double DI until one starts losing strength?

Just use one Di vessel, until the reading goes above 0 . Then add your new vessel after the first one . Keep checking the reading out of your second vessel . As soon as it goes above 0 putvthat as your first one . Bin the resin out of the original one . Put new resi in and make that your second one . Keep shaking em both up . Repeat for the rest of your window cleaning life . 

I have a twin set up of 2 eleven litre Tanks, I only ever replace Resin in one Tank, then reverse them, incidentally I always used to use Tulsion 115 I replaced the Resin every 4 weeks when the Resin shortage was on I nipped up the road from me and bought a bag of Unger it's lasted 6 weeks and still going. 

Yeah mate unger Premium lasts a lot longer for me too.

Elliot what's your reading out the tap in Dewsbury....mine's gone up here in Batley from around 80/90 to 110/120.