If I fly from the United States to Beijing and get a 72 hour free transit

then fly from Beijing to a third country (for a few days), and fly from that third country to Shanghai, can I get an additional 144 hour free transit? Or is there a limit to the number of visas you can be issued in a certain period? Here's what my itinerary would look like:

SFO - PEK
PEK - SGN
SGN - HND
HND - PVG
PVG - SFO