Roll up to CX checkin at T1, show both tickets and ask for bags to be tagged through and to get all 3 boarding passes.

No clue if you need to re-clear security- probably not..but worst case, you re-clear security and get back to the same place.

If for some reason you get 1 boarding pass SIN-HKG..expect the miles for that and not SIN-BKk-HKG..