Two 144-hour visa free exemptions ?

Hello !
I will arrive in Beijing and leave the next day toward ulanbataar, Mongolia. I plan to ask for the 144-hour free exemption at the airport.
A few weeks later I will come back to Beijing and spend a few days there (less than 144 jours) before flying back to Geneva through Dubai.
Is it possible to apply twice for this 144-hour free exemption or is there some limit of any kind?