Did you purchase the phones on the DPP? Does your receipt show the 1/2 off phone?
We did the same thing - my receipt shows one at full price, one at 1/2 off. Our DPP payments are $27.xx and $13.xx - one is half the other. My bill reflects the correct device payment plan amounts. I could see what my expected bill would be the same night we bought the phones, prior to purchase. What does your receipt say?