The only potential hurdle to be aware of with AA, and it shouldn't be a big problem if you know it might happen, is that gate agents expect boarding order to be printed on your boarding pass. If yours says Group 4 or whatever, their initial reaction (if they're enforcing boarding order at all, which isn't always the case) will be to shoo you away. So have your other card out and be prepared to say something like "I know, I'm boarding now because of my oneWorld level via Qantas [or whatever], it doesn't show up when they print boarding passes."