How to Mirror Selected Screens
The problem was that Sal could only see the MacBook Pro and the iPad on the podium. For anything that happened on the projector, he had to peer into the distance to work from the projected screen. That proved difficult, but he soldiered through the presentation.
Afterward, Ed Marczak, a speaker and advisor to MacTech Conference who wasn’t in the room during the setup time, shared a little-known tip that would have made things easier on Sal. It turns out that if you have three (or more) screens connected to a Mac, you can mirror two of the three by Option-clicking one and dragging it on top of another. This does exactly what you’d expect. The Mac acts as though it has only two screens instead of three, and the same image appears on the two that are mirrored, with the third being separate.
