Codeshares cannot be booked on their own. They have to be booked as part of the long haul however if the codeshare isn't available, then agents have to book the flights as AC as CX/AC have a joint fare agreement.
Honestly that is what I did when I was a TA. But nowadays, people think that they know better. The GDS simply will not allow a codeshared flight to be book independently if not connected (married) to the long haul sector.