F business visa

There is no way to extend the stay for each entry, if you apply for visa extension when the 2 months stay duration will be used out, you can get another 30 days stay duration, but your multiple entry visa will expire, it is not a good idea for you extend the stay duration in China, you can go to Hong Kong when the 2 months stay duration is up, then return to Mainland China for another 2 months stay. You can return to Mainland China on the same day when you go to Hong Kong.