How To Split Audio Between Two Headphone Jacks?

I think what you want is to split a stereo headphone jack so that two people can listen to the same sound, both of them in stereo.

There are many inexpensive two-for-one adapters available that will do this. Just be careful: some "splitters" will break up the stereo sound so that one person hears the left channel only, while the other hears the right channel only.