Beijing-Hong Kong- Beijing

Hi, I am planning a trip to Beijing for 3 months, but my visa is validated for two months (Netherlands). So I would like to plan a return trip to Hong Kong and go back to Beijing by train.

Is this legit? And if so, where do I receive a validation that I have been to Hong Kong, so I can spend another 30 days in Beijing?

Kind regards,
Maikel
(Netherlands)