I'm not sure on your question about WB's trip, because it depends on routing. From NY, AA most often uses their own seats for the ocean hop, but does use some BA seats from Newark. Other cities...not sure how many BA seats used in that direction. And sometimes it's a matter of class. For instance, last year flying home from Paris, there were AA seats in business class available non-stop, but economy was only available on BA Orly-London-Newark.