Can I claim the 72 hour visa twice in a fortnight?

I am a resident and citizen of the United Kingdom. I would like to visit both Shanghai and Beijing whilst traveling to Japan. Is it possible to fly to Shanghai and spent 72 hours there, travel to Japan then on my return to London spend 72 hours in Beijing?

Failing that would it be acceptable to go (on my return to the UK) from Japan to Beijing claim a second 72 hours (in a two week period) and then go to Hong Kong/Seoul and then back to London?