You agreed to a 24 month comitment you didn't commit to. There is no contractual obligation on service, pay DPP off early and you sacrifice any promo credits on the line receiving them. It's always been this way for those type of promotions.
You didn't pay off both devices with one payment because you have two agreements. Verizon gives you credit on the second one to cover it for 24 months. Never, ever pay off an agreement early getting credits.