Is that possible to use the 72 hour transit 2 times so I can visit the 2 cities?

I am US Citizen and here are my trip details ( my intention is to avoid the L type (save $500 total courier fees for me and Wife).
1. Atlanta to Shanghai(Delta) spend 2 days here (use 72 hr transit)
2. Leave Shanghai to Hong Kong for 5days (Hong Kong Airlines)
3. After 5 days, leave Hong Kong back to Shanghai (Hong Kong Airlines, use 72 hr transit again)
4. after 2 days leave Shanghai back to Atlanta..(Delta)

Is this doable or it looks like I need the Chinese entry permit Still. Otherwise I may just go to HongKong and cut out the whole mainland piece of it.