tibet visa application

I just want to apply a visa for Tibet and join my wife in Chengdu as she will arrive with a tour 21.082017
I am living in Hong Kong and have a valid china visa up to October 2017 and be a Hong Kong resident.

I just want to take a flight from Hong Kong to Chengdu 21.08.2017 and return to Hong Kong 01.09.2017 therefor my question whether I just can apply and get a Tibet Visa during that period. In fact I join the tour too together with my wife which had been booked already in Germany through "MARCO POLO" travel agency.

thanks for your advise
best regards
Norbert