It just seemed like the OP specifically wanted to go to SE Asia, so I noted that there's no need to buy a RT -- the AS miles will get him there, and AA miles can get him back on the same partner. Of course you can also collect Amex or Chase or whatever pts and transfer to a different airline to get back. The real point here is that not having enough for a RT is not a deal-breaker -- just use two different programs for two 1-ways.
I'm also not sure where the 120k number comes from for the round trip.
Personally I'd suggest that the OP use 50k miles one-way on CX J (not that I'm a huge fan of CX but he has 52k miles so it seems like the obvious choice) and collect AA miles (or some other type of miles/points) for the other 1-way.
Another option, if collecting AA (or other) miles is not easy, and the OP is short of miles in general, is to just buy the remaining 48k miles for $960-ish during a 40% bonus mile sale, and spend 100k on the round trip in CX J. It's really not a bad deal at all. Don't forget the free stopover(s) in HKG if those interest you.
All this assumes that there is CX J availability, of course.