You have to exit China when the stay duration will expire, otherwise, you will overstay the visa. You can go to Macau or Hong Kong, then return to Mainland China for your second stay duration. If you really don't want to exit China when the stay duration will be used out, you can go to the local Exit & Enter Department to apply for visa extension, you will be issued a new visa on your passport with more stay duration, your present double entry automatically expires.