I would not suggest that you cut it close assuming this will work, but I think there's a strong likelihood it will: When you get to the priority security line, show the onward BP - they don't inspect them closely IME and are unlikely to notice its for the second leg. Won't help with boarding etc but will at TSA. For check-in I'd think you could do priority, especially if the second leg is intl but might be wrong on that as I've never not had EXP on AA (UA convert), again I'd give it a go beg for forgiveness rather than permission.