Trade in 2 iphones

If you go to the Apple Store in-person, you can trade one of the phones directly towards a new phone and Apple will give you a Gift Card for the value of the 2nd phone, which you can use as a form of payment towards the new phone.

Yes, they don't care about the box or cables. You will have to be able to unlock the phone and demonstrate it is in working order and any dings, dents will be reflected in a lowered value.