If you add a XBee and shield to the Arduino, and data that the Arduino pus on the serial port will also be transmitted over the air by the XBee. Anything that the XBee receives over the air will be put on the serial port, just as though it came from the PC.
So, add an XBee and shield to the Arduino, and an XBee and USB Explorer
to the second PC. Anything sent to the first PC will also be sent to the 2nd PC.
It is possible to have each PC send ID information along with data, so that the Arduino knows whether data came from PC 1 or PC 2, if PC 2 will be sending data, too, and if it is important to know which PC the data came from.