To connect and use two Bluetooth headphones simultaneously with a Mac, follow these steps:

  1. Ensure both Bluetooth headphones are paired with your Mac.
  2. Open the Audio MIDI Setup app, found in the Applications > Utilities folder.
  3. Click the plus (+) button at the bottom left of the Audio Devices window and select "Create Multi-Output Device."
  4. In the Audio Devices list, tick the boxes next to both headphone pairs you want to use.
  5. Select a Master Device from the drop-down menu.
  6. Enable "Drift Correction" for the secondary device(s) to ensure synchronized audio, especially if using Bluetooth headphones.
  7. Go to System Preferences > Sound (or choose from the Apple menu > System Settings... > Sound), and on the Output tab, select the newly created Multi-Output Device or "Aggregate device."

Now both headphones should be receiving audio from your Mac simultaneously. The procedure should work for two Bluetooth headphones, and also for a combination of Bluetooth and wired headphones, as long as they are connected to your Mac.