What are headphone splitters used for?
Headphone splitters are a device you can use to plug into your headphone jack and then attach other headphones to it. Headphone splitters make it so you can share music with more than one person at a time, with everyone using their own set of headphones.
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.