they changed the rules (at least for Americans) as far as multiple entry visas, a couple of years ago. Now, you get the choice of one entry, two entries, or multiple entries (sounds like something you would negotiate with a WG
). It used to be that you had to provide a business reason with company letterhead in order to get the M (multiple) visa, but not anymore. Two entries and multiple are pretty much the same price, so always go for the multiple.
If you find yourself in HK without a China visa and you need to go, you would need to go to the Chinese consulate building in Wanchai over by the Renaissance Hotel. For an American, which probably applies to other countries as well, you could get the visa back the same day if you show an emergency need. An emergency need would be something like visiting a relative who was about to die. Otherwise, you'd have to wait at least one day at the earliest for your visa. I say this because it used to be pretty convenient and easy to just go to any CTS travel location, turn in your visa in the morning and get it in the late afternoon. Not anymore.