Audio cable "joiner"?

It sounds like what you are after is a "Passive stereo mixer", which just use resistor summing networks to combine the 2 signals. They're pretty simple to make and there are instructions online which describe what to do. I've used them before to combine stereo signals to mono at line level, but I've never used them at headphone levels.

It's called a "mixer" or an "audio mixer". You can't just connect the audio wires in a Y since this will create a path for current to flow from one output device to the next, which could damage them.

If you are trying to combine the audio from two separate computers into one set of headphones, there is an alternative to buying/building a mixer. Here's what I did.

You can plug the audio line-out from computer A into the audio line-in on computer B, then plug the headphones into the headphone out on computer B. You will likely need to tweak the audio mixer settings on computer B to get the right sound level balance.

I'm late to the party, and you've already selected an answer, but I think this would solve your problem and doesn't require a computer.