it depends on the codeshares - in some cases the codesharing partner is permitted to sell them as stand alone - in some cases they must be specifically 'tied' to a flight on the codesharing partners own metal . As I am no longer in the industry I no longer have access to which flights are or are not permitted - but if a flight is listed as not permitted in OW explorer rules - or , if when booked in the CRS it comes back th the booking agent with a response such as "online connection only" then it is a risky strategy to make a reservation as a 'stand alone' sector - often the airline stands to be fined if it breaks the rules - and therefore there is a strong financial incentive for the airline to be vigilant - in other cases it may be a codeshare that neither airline is concerned about and it may easily 'slip through the net' or even be fully permitted - having said that I am surprised that CX would book HKG AA NRT AA HKG as stand alone sectors as I don't think AA have stand alone traffic rights on that sector - was there no AA flight in the itinerary NRT-USA vv ??