Did you put the devices on a payment plan? Pay for them outright?
We did the same thing on Nov. 11 - 2 Pixel's, buy one get one half off on lines eligible for upgrade. From Day 1 we have seen the device payment plan reflect that - one payment of $27.xx and a second of $13.xx. Over the course of 24 months we will pay half for one phone and full for the other. The "buyout price" also reflects that one is half the cost of the other.
What do your receipts show for that days purchase? (You can view the docs on line if you don't have the store receipt). What does your billing show that you are paying for each line?