Can you have the 72 hours transit visa if flying on separate tickets?

Hi, I'm flying into HKG>Shanghai on the 26SEP arriving at 1620pm.
Then leaving on a separate one way ticket Shanghai>Busan (S Korea) on the 29SEP at 14.10pm.
Is the free transit valid for 2 x one way tickets or must it be on one ticket or booking?
Any help much appreciated.