having two DI vessels on a system has no benefit at all, you may as well have a 22" or a 32" vessel. if you have two all that happens is that the first vessels resin is exhausted first then when that has no more capacity to purify the resin in the second one will then do the work.
By having two small resin vessels, you will have to change the vessels less often before your TDS reading rises above 000 but when you do change you will need to change both.
I advise you get a single 22" or 32" vessel and save yourself sometime and only need to change 1.