Double entry visa query - duration of stay

Hello,
My wife and I have recently received our Chinese double entry one and have been provided one with 30 days for the duration of stay.

Can you confirm if this 30 days refers to each visit, I.e. 30 days for the first entry, and 30 days for the second entry (60 days total), or for the total period from time of entry, I.e. would we have to leave PRC 30 days after first entry (even if we have left and come back in for the second entry already) and not be allowed back in without a new one?

I hope he question makes sense!

Thanks,
Alan