you can use a splitter, they are very cheap (i am in uk but you can find them wherever you are for the same price with conversions and all)
http://www.cdlmicro.co.uk/html/show_product.php/pid/41
that is it, you plug one into one, the other into the other and away you go. You will need to turn one down though, unless you want sound from both, because sound will always come through the speakers and the headset