If you are LOGGED INTO your account, you can pay with the app using multiple gift cards.
If you just try to use the QR code and pay, you are correct, you can only use 1 gift card.
Login to your rewards account and just do it yourself.
I cut/paste the numbers from an email I have so I am not manually entering 16 digits.
I never want to give my server the gift card number if there is a $100 balance. Not saying they all steal, but why give someone something that could be stolen and used without your knowledge or permission.