Q5: How can I connect two PCs to each other with USB?
A5: You need a specialized USB peripheral known as a USB bridge (sometimes called a USB to USB adapter) to do this. Anchor Chips and e-Tek labs, among others, make USB bridges.
Q6: You mean I can't make a direct cable connection like a null modem?
A6: Correct. In fact, if you try this with an illegal A to A USB cable, you'll short the two PCs' power supplies together, possibly destroying one or both machines or causing a fire hazard. Even there were no danger to the machines from the problem with two power supplies, there still wouldn't be any way to get the two PCs talking to each other, since USB doesn't support that particular kind of communication. A reasonably priced solution to handle this need is the USB bridge.
Taken form
http://www.usb.org/about/faq/ans5/