Free Transit 2 times on 1 trip?

We are planning to fly from UAE to Shanghai (Staying for 4.5 days) hoping to utilize the 144 hr free stay and then travel to Hong Kong. Then after 4 days in Hong Kong, would like to travel to Beijing (staying for less then 72 hrs) and fly on back toward Italy. Can we do the free transit policy twice in one trip? will they look at a previous stamped page and give us a problem?