how to use 30 days 2 entry china visa?

hi there! i am a Filipina i have a brasilian boyfriend working in china,i wanna ask bout using the 30 days 2 entry china visa as the agency told me that once i enter china its already counted as 1 entry so i still have another entry available. My question is if i go out China earlier before the 30th day of my stay, can i still be able to get back inside china to use the other entry and would that be mean that i will have another 30days to stay inside China? Because this is what the agent explained to me so i wanna make it clear and seek some advice from you guys.

I am planning to travel to Brasil on February 6th 2012, but i will get back first to China this January 6th 2012,so i really want to clear things up using my it if i have to make extension first before going to Brasil so when i get back after 10-15 days tour in Brasil then entering in China will be easier.

Any advice any suggestion will be truly appreciated!
GODBLESS! =)