i'm assuming here that you have an analog headset (3.5mm) and not usb.
if so, if you want to play one or the other... a simple A/B switch would work. cheap.
if you want to play one or the other or both at once... a mixer is what you want. fairly cheap.
if you have an optical headset... mixers are expensive so dont bother.
if you have usb, you could use a usb switch although would only be able to A/B not A+B