Sometimes it's nice to listen to a movie using headphones. It's also nice to watch a movie with someone else. Now, you and a friend can use headphones to watch the same movie on your Mac.
Let's assume you are using AirPods, connect both AirPods to the Mac:
Next, open the "Audio MIDI Setup" program. It's under Applications/Utilities.
Press the "+" on the lower right. Create Multi-Output Device.
You will get a new device. Click Use next to each AirPod:
Now select it using Sound Preferences.
You will be able to hear sound from on each device. Start your movie and enjoy!
Note: If either AirPod disconnects then re-connects, the Mac will automatically switch to the AirPod. You will need to re-select the Multi-Output device to get sound on both devices again.