CX can operate at both (or either) if they decide to do so, though their public statements on the matter indicate they plan to stay at PEK. However, now that CA has been allocated 10% of the slots at PKX, things could change.

As as far as I know, CX doesn’t have a lot of transfer traffic in BJS (on CX plated tickets, anyway). Isn’t the majority O&D traffic? If so, what’s the benefit of them serving PKX? It’s not closer to the city, so it doesn’t seem like it’ll be a more convenient airport for O&D traffic.

It seems likely that OW carriers may largely operate from PEK (AA definitely will) and there are rumors of an OW branded lounge opening at PKX. If enough OW carriers move to PKX, that could give CX a reason to serve the new airport.