Using two resin bottles will make little or no difference to the amount of resin used. Resin will remove a set volume of impurities, so it is mathematically possible to calculate how much pure water your resin will make if you know your starting TDS. Once the resin is depleted it will not remove any more impurities.
As most others have said, the only sensible way to purify if you have more than about 50-60 TDS input is to use an RO
I use two and it does save money. Mind you my TDS is only about 57 ppm.
If I used one vessel then I would have to bin the resin as soon as the TDS level got to 5 or 6 ppm.
With two vessels, I can carry on using the one giving out 5 or 6 ppm as the first vessel, as 5 or 6 is a hell of a lot less than my tap water at 57ppm.
When the first vessel reading is the same as my tap water there is nothing more to be gained from it, and I bin that resin, I then continue using a single vessel until it reads 5 or 6 ppm, I then put the newly filled vessel behind it.
I now have 5 or 6 ppm TDS running into the fresh resin again.
With 57 TDS, tap water, I get about 3 months work out of a single 25ltr bag using about 200 to 250 liters a day on average.