Trade in

What you could do, is trade the second phone in before you buy your new phone. You can trade something into Apple for an Apple Gift Card anytime you want. Once you have the Gift Card, then you might be able to use it towards the purchase of the new iPhone along with your trade. You would want to purchase the new iPhone in store as opposed to online.

You can trade one phone towards the purchase of a new phone. If you want to trade another device, Apple will give you an Apple Gift Card, which you can use for other Apple purchases.

You're welcome.