Is it possible to enter China mainland (on transit visa) twice?

For example, departing from Seoul to Shanghai, staying there 2 days, then flying to Hong Kong for a few days, then flying (or taking the train) to Guangzhou for 2 days to visit friends, and flying back to Seoul from Guangzhou? Each independent stay would be less than 72 hours, but I'm curious to know if it's possible to use the G type twice.
Thank you.