2 reservations on same day
 

if i have two reservations on the same day, 1 for the hkg-jfk morning flight and one for the evening, will cx see this and cancel them both out?

Maybe if the loading on the flights is tight. And if they do spot it, then depending on what class you are in and what your CX/OW status is and whether they have your contact details then they may or may not contact you to ask which one you wish to cancel. But this is only likely to happen IME (but as someone with CX status) within the last few days before the flight.

You should call them to confirm that the flight will not be automatically cancelled out. They'll put a note on the booking.

BTW, why did you book 2 flights anyway?