Is it possible to use iPhone Tethering with two computers at the same time?

As from experience, you can only do it through one connection and USB takes dominance over Bluetooth connection. If you know how to do Internet Connection Sharing (ICS), you can easily share your iPhone's connection through your computer's wireless device.

"If you can connect the two computers by some other means, you could share the Internet connection from the tethered computer to the other one.

For example, if your tethered computer is a Mac, you can enable Internet Sharing via the Sharing panel in System Preferences. Then you can share the connection via wired ethernet, FireWire, computer-to-computer (ad-hoc) Wi-Fi, or possibly Bluetooth.

I'm fairly sure a similar option is available on Windows though I can't tell you precisely where to go clicking."