you only need 1 vessel to make pure water dude, the second vessel is for resin economy.
For example:-
1 vessel reaches 001/002/005 etc tds. now you would have to throw it away & refill with fresh resin if you only had 1 vessel, wasted resin
So, instead, you now fill the second vessel with fresh resin & put it after the older vessel which is at 001/002/005 etc tds.
Now you have tap water first reading 85ppm, then the old vessel reading 002 etc, then the new vessel reading 000.
So your old resin which you would have thrown away is now pre-filtering your fresh resin, like putting RO water through your new resin. you just keep this method going then.
DOWEX resin will hold a low tds in the old vessel for ages, so pre-filtering will last the life-span of the new resin. there is also a trick to make your resin go back to 000ppm when it starts to rise but more on that when you need to know.
Do you now see why it is much more economical to use 2 x DI vessels in sequence?
tony