Unfortunately I don't think that there's any way to in fact link two separate tickets together to avoid the APD. If the two flights are on separate PNRs I don't think that there's any way to accomplish this. On two recent occasions I have approached BA regarding this issue, and on neither occasion were they the slightest bit helpful in regard to avoiding the APD. One AA RTW desk customer service agent did some investigation for me regarding this, and after a very long offline discussion with his Fares desk, he asserted that if both tickets were issued by AA, and the departure one had not yet been issued, that he could figure out a way to accomplish not charging the APD on the yet-to-be issued departure ticket. [I never took DeWayne up on his offer...in part because I was pretty certain that his assertion was only theoretical, and that I would probably end up spending a fruitless hour on the phone with him and his Fares desk....].