If I enter PRC from Australia on double-entry tourist visa can I apply for extension after 30 days?

I plan to enter China (beijing) on a double-entry tourist visa. After about 10 days I plan to go to Hong Kong for a week and then return to Beijing. I will then want to extend my tourist visa for another 30 days but I have been told that it is not possible to extend a double-entry tourist visa. Two questions:
1) Is a tourist visa applicable for only 30 days?
2) Can I extend my tourist visa if I originally entered on a double-entry tourist visa?

Thank you