No, the USB headsets act as an audio device ... and Windows will only output to one audio device at a time.
But if the PC has a standard audio adapter (either built-in or an add-in card) with a standard audio output jack, you can use one of these Y-adapters to connect two standard sets of headphones:
http://www.monoprice.com/products/product.asp?c_id=102&cp_id=10218&cs_id=1021802&p_id=667&seq=1&format=2