Singapore - Hong Kong (48hrs) - Xi'an (48hrs) - Singapore (Canadian Passport)

Hi,

Is the 72 hr free transit the total time spent, or is it time spent in each separate visa-free city? I am travelling on a Canadian passport from Singapore to HK for 48 hrs, then flying to Xi'An for 48 hrs, then back to Singapore. Do I need a visa for this trip, or do the 2 separte destinations count as 2 separate visa entries?

Thank you!
Phil