The only way you can hold another reservation of the same flight is if its booked with a different flight number, ie codeshare. But since CDG-HKG is not a AA code-share with CX, that cannot be done.

If you need to book with CX, then you must cancel the AA held reservation first otherwise you risk both reservations being cancelled. Once the AA held booking is cancelled, that seat goes back to general inventory to which you may then book with CX.