You want to tie a device to a purchase. And as I told you, you can only trade ONE device towards one phone. You want to buy two phones. But you have 3 phones and an iPad you want to trade. So yes, if you want to trade a phone towards a new phone, you'll have a device tied to a trade in. And if you want to buy two phones, you'll have two devices tied to two device purchases. That leaves another phone and an iPad you can't actually use to trade towards either iPhone. So I suggested trading them in to Apple, which won't be tied to any purchase and Apple will give you a gift card.
The question is whether you can use the gift card as partial payment for either of the phones you want to purchase, knowing each of the phones you want to purchase will have a trade tied to them. That's where you need to ask Apple.
I don't know if I can be any more clear?