USA - South Korea - Shanghai - Hong Kong

Hello, we are planning to fly with Korean Airlines from California to Shanghai (stop in Seoul for 3 hours). After Shanghai we are planning to go to Hong Kong. Can we use 72-hours visa for Staying in Shanghai for 2 days (we want to go out of airport and visit city)? Do we need to obtain oen in advance or we will get permit upon arrival?

Thank you in advance!