That's what 'married segments' mean. The flights have to be sold as a pair and not singularly. If you want that seat from HKG, then you have to buy it as HKG-LAX. You cannot marry the two on two seperate bookings. You have to basically do all bookings on one PNR.

You need to waitlist for your preferred flight on your original PNR and cancel the other booking which you are holding.

That is why CX chose AMADEUS GDS. Airlines using this system also have the married segments situation - ie LH, OS, SQ, BA, QF.

There is a way to choose a single segment from availability but it takes a travel agent to do it.. so you have to next time allow a TA to book your flights.