The method described below should work regardless of whether you're using one wired pair and one wireless pair of headphones, two pairs of Bluetooth headphones (i.e. two sets of AirPods), or even several pairs.
How to Output Mac Audio to Two Audio Devices
- Make sure that the headphones you want to use together are paired with your Mac over Bluetooth and/or connected via the headphone jack.
- Launch the Audio MIDI Setup app, located in Applications/Utilities.
- Click the plus (+) button at the lower left of the Audio Devices window and select Create Multi-Output Device.
- Right-click (or Ctrl-click) the Multi-Output Device in the list that you just created, and select Use This Device For Sound Output. (You can also opt to Play Alerts and Sound Effects Through This Device from the same menu.)
- Tick the sets of headphones that you want to use in the Audio Device list. (If one is a wired pair, tick Built-in Output.)
- Select a Master Device in the drop-down menu.
- Tick Drift Correction for the slave device in the Audio Device list.
- Launch System Preferences (select -> System Preferences... from the menu bar) and open the Sound pane.
- Click the Output tab and choose the Multi-Output Device or "Aggregate device" in the list, and you should be good to go.