They could use an Airbus 340 instead.

I guess it's cheaper for CX to stick with a single base at SFO than to split staff betweeen SFO and SJC, though. I'd expect a number of efficiencies from consolidation (everything from fewer signs to put up to fewer baggage handlers to contract with). So I wouldn't expect to see CX's San Francisco-area flights split between SFO and SJC unless there were an awfully compelling reason to do so.

(I guess the possible reasons would be: Can't get more slots, gates, etc. at SFO. Can't get good enough AA feed into SFO. Anything else? Landing fees, gate fees, parking space rental, etc. probably not enough to matter.)