Agent in HK?
 

Hi -

I've normally been booking flights on CX for the HKG -> LAX, SFO, JFK leg and then booking another flight from sites like kayak.com from one of thos cities into another US city.

If I try to book on CX's website.. it ends up costing me double of just the first leg!

There must be others who have this issue.. do you just issue two different tickets like I do or are there good agents in HK that you use to facilitate this at a better rate?

The CX website is generally not a good place to make bookings. At the very least use a website like Travelzen so you see better than B fares.

Look into aa.com and booking the AA codeshare on the TPAC flight, or alternatively try pricing the itinerary with the CX flight number on the TPAC flight. This is how I commonly book flights originating in the US, though I'm not sure if the pricing is as beneficial if originating in HKG. Also, booking an AA codeshare may restrict whether you can use miles for upgrading, etc.

It's OW so I doubt you can upgrade an AA codeshare with miles. Regardless, upgrading with miles is a bit of a joke on CX anways, it's restricted to a few very high fare codes. Other than that though it should be fine. Fortunately earning CX miles by flying AA is pretty generous.

Despite moving to HKG I haven't really looked around for a good TA yet, I do most bookings myself so I can't help you on that front.