If you arrange a trade as part of an online purchase, the trade amount will be credited to the payment method that you used for the purchase -- but I don't know of a way to do that with more than one device while making an online purchase. You may want to contact Apple Sales support to verify -- call 1-800-MY-APPLE.
Another option is to trad for Apple Gift cards ahead of the time you make the purchase - see: Apple Trade In - Apple
You can then use multiple Apple Gift cards for a single purchase. See the following from Payment, Financing, Refunds and VAT - Shopping Help - Apple
You can use up to eight Apple Gift Cards at a time when you purchase online from Apple, or if you’re placing an order by calling 1‑800‑MY‑APPLE. You can use your gift card(s) with one credit card and your Apple Account Balance to pay for your purchase.