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How to mirror only some displays

  1. Go to System Preferences, Displays, Arrangement.
  2. Click the Mirror Displays checkbox.
  3. Hold down the Alt key to drag a secondary display onto the primary (the one with the menu bar), to mirror them (in which case the menu bar will show on both). Alt-dragging one display onto another to mirror them can also be done for two non-primary displays.
  4. If you have 4 displays and want 2 pairs of mirrors, you have do the primary and the secondary first, then the 3rd and 4th, not the other way around.

Source: https://create.pro/how-to-mirror-specific-displays-in-os-x-mirror-some-but-not-all-of-your-monitors-on-an-apple-system/.