Extend two Entry Visa: Stay longer in China than 30 days at once without having to leave china.

I have 2 entries. Each stay is 30 days, so I will be able to stay 2 x 30 days. My problem is that I don´t want to leave after 30 days, because of the travel route.
So I was thinking about extending it . Is that possible?
Would that mean that I get a new one for longer, but then the 2 entry isn´t valid anymore?
Thanks!!!