Can I change my two iPhones for a new one?

Just answer a few questions either in an Apple Store or online (https://www.apple.com/shop/trade-in - Click on Smart Phone icon) regarding the brand, model, and condition of your device. They will provide either an estimated trade-in value or a simple way to recycle it.

If you accept the trade-in estimate in the store, they will give you instant credit toward a purchase.

If you accept it online, they will send you a trade-in kit (at no cost to you) so you can send them your device. Then, once they receive your trade-in device, a team will inspect it and verify its condition. If everything checks out, they will either refund the purchase method you used to buy a new Apple device or they will send you an Apple Store Gift Card via email.

If the condition doesn’t match what you described, a new estimated trade-in value will be sent to you via email. You’ll have the option to either accept this revised quote or reject it. If you reject it, your device will be sent back to you at no charge.

You'll want to check with an Apple Store to confirm. Based on what I know I believe they give you the option to use the credit immediately on a new device. However, I don't know if you can get immediate credit for multiple devices.

You won't get any money for the iPhone 7 but Apple will recycle it for you so it doesn't end up in a landfill.

You can do the trade-in through that link and receive an Apple Store Gift Card or you can do the same at an Apple Store.

I’ve tried a sample trade in value for an iPhone 7 with a cracked screen and the value goes to zero. If it otherwise works, fixing the screen may provide more value as a gift or if you’re willing to sell it privately.