using 72-hour free transit in Beijing twice

Hello,
Is there a cap on how many times I can use the 72-hour free transit? I would like to fly from U.S. to Beijing, then Japan (within 72 hours). I would then like to fly from Japan back to Beijing and then to Hong Kong (also within 72 hours). Is this possible to do within a span of two weeks?
Would a roundtrip ticket from Beijing-Japan-Beijing not be allowed even if I plan to fly to Hong Kong 72 hours later?