Can I leave the Shanghai and return 2 weeks later on a single 144 hour visa?

Hi,
I have planned to fly to the next February. I plan to spend three days there then fly to Vietnam for 12 days, coming back to SH for a final two days before my return flight to the UK.
I plan to stay within the 144 hours limit but can I leave Shanghai and come back again on the same visa? ie is the hours cumulative or a new visa required?
Thanks