How many 72 hour visas can you have in a month?

Hello I fly to Shanghai (en route London to Cambodia) - have 6 hours where i'd like to explore outside using the fast train to the city, and so need a 72 hour transit, but i then return to Shanghai 2 weeks later, as part of my return journey to London from Cambodia, and have another 6 hours where i'd like to stay at a airport hotel - will i be able to apply for a second 72hr transit in such a short space of time?

Thank you very much.