I want to run two audio sources through my headphones
Yes you can combine 2 audio sources into one. Usually that's done in a mixer, but if you really only want to add 2 audio lines together and don't care too much about audio quality, a Y cable like this should be lightweight enough to work with your setup. Make sure you get a cable that has the male and female parts the right way around, and that you don't get a stereo splitter, or a cable that just separates audio in from audio out - it's easy to buy the wrong thing.