It is not possible to use more than 1 M-Audio USB Audio Device attached to the same computer.  M-Audio does not currently support the installation of drivers for multiple M-Audio USB Audio Devices on the same system.  There are driver file names common to all M-Audio USB Audio Devices, and installing drivers for a second device will overwrite files necessary for the currently installed device, and can result in this device no longer functioning properly.  If you would like to switch between diferent M-Audio USB Audio Devices, you must uninstall the drivers for the first device before installing and connecting the second.