You should be able to get both boarding passes in HKG as well. This is unless your name is on some kind of blacklist or something and you're "randomly" selected for special screening. Yes, it's happened to me. Then CX will not be able to issue your onward BP (but will still tag bags thru).
Just remember that you MUST collect your bags in LAX and put clear customs and put them on the transfer belt. This is even if your bags are tagged through.