Use free transit service twice in two weeks?

Hi. I am flying Vancouver-Beijing-Ulan Baatar then 10 days later I am flying Ulan Baatar-Beijing-Vancouver. Can I use a 72 hour visa at the start of the trip when I arrive Beijing from Vancouver on my way to Mongolia, and then use another 72 hour transit at the end of the trip when I arrive Beijing from Mongolia on the way back to Vancouver? Thank you!