Double entry on an X2 type - is it possible?

I have a 5-month visa to study in Shanghai (X2 type). It is single entry as I was told at the Chinese Visa Application Centre in London that they could not issue double entry for this type. However, I was planning to leave China four weeks into my stay to go to a wedding in the US. I understand I can obtain permission from the local PSB office in Shanghai. Can anyone give me more information as to likelihood of this being granted, time it takes, what I need to do etc?
Many thanks!