If you are just using standard headphone cabling, any "dual headphone" adapter will work fine. It's not perfect, and there's no safety in that if one device breaks, it could break the OTHER device as well. A mixer is the proper solution, but likely as not it's not something you really want to mess with.
Search in Google for 3.5mm splitter and then look until you find the correct m/f combination you need. Here's one you can just click and buy: http://www.amazon.com/Stereo-1-Female-2 ... B0009JFF7U