QR should be able to provide you with a separate PNR for the AT portion of the trip. Then just go to manage my booking on the AT site using your last name and the AT record locator. You should be able to do seat assignments via “additional services”. I just did the same on a trip booked through AA. If you have OneWorld elite status, QR a should be able to assign a seat. That also worked for me on my AA itinerary. Hope that helps.