You can definitely trade in multiple devices in the store and for each you'll get a gift card to apply to another purchase (or use the gift cards later). But if an online purchase and multi-device trade-in doesn't work, you may need to call or chat with retail support. It's so much easier and faster in the store if you're near one.
I assume you've already used the standalone trade-in tool to see what the estimate would be.
Trade in your iPhone, iPad, Mac, Watch or any other device for credit towards a new one, or recycle it responsibly for free with Apple Trade In.
www.apple.com