Instructions
Turn off the controller. Hold down the "PS" button until a menu pops up on the first PS3. Select the "Turn Controller Off" option.
Plug the small end of the USB cable into the top of the controller. Connect the larger end of the cable to an open USB port on the second PS3.
Turn on the second PS3. Once the system finishes loading, press the "PS" button on the controller. This will assign the controller to this PS3.