Can you trade in TWO devices on a single device purchase like you are proposing?
Apple will only let you do a 1-for-1 device trade-in credit toward immediate device purchase.
However, you can trade in multiple devices and get an Apple store gift card for each. You can then use the gift card(s) to purchase a device later.
The iPhone 16e is worth $310 whether it's for an Apple gift card or for credit toward the immediate purchase of a device which is kind of unusual. From past trade-in's I've done, Apple would usually give you a little more (like an extra $10 or $20) as an incentive toward an immediate device purchase vs putting amount on an Apple gift card.
For example, when the iPhone 17 came out and I was contemplating trading in my iPhone 14 for it, Apple offered something like $240 toward an immediate device purchase. But if I wanted an Apple gift card to use later, I would have gotten something like $220 instead.
So, trade in two iPhone 16e for two $310 Apple gift cards. Use the two gift cards to buy iPhone 17e.
If you have another address to use, you could buy another two iPhone 16e, trade four of them in, and use the Apple gift cards to get an iPhone 17 Pro Max for like half off.
Buy four iPhone 16e. Your out-of-pocket cost is 4 x ($99.99 + 1 month of $55 plan) = $619.96
Trade-in gets you $310 credit x 4 = $1240
iPhone 17 Pro Max 256GB is $1199