[waydabber]

Of course, you can configure everything as usual under System Preferences/Displays.

If you have a 2+ display setup for example, you can create a Dummy, mirror it to one display but leave the other display unaffected. Or you can create 2 dummies, mirror each one to a separate display.

Every display can serve as a Main or Extended display, as well as a Mirror for some other display. Therefore a mirrored set can be Main or Extended as well. A typical use case is to use your MacBook display as an Extended display alongside your external display which is a Mirror of a Dummy that is set as Main display (providing all the fine grained HiDPI resolution options).

Hope this helps!

[terramex]

Is this a macOS 12 feature? I am stuck on 11 for now and I do not see any Main/Extended setting.

[waydabber]

The app is compiled for Big Sur but I did not test it and received conflicting reports whether it works or not. Please try it and let me know! Thanks!

[terramex]

I tried again got it working this time. I turns out that in versions below 12 the way to set up mirroring of only 2 out of 3 displays is: "Press and hold the Option key and drag one display icon onto another display icon to mirror those two displays.". 10 years of using Mac every day and never knew it was possible.

It is pretty buggy though, "Arrangement" tabs stops working and I cannot change relative screen positions anymore, so to get it kinda working I had to set my real and dummy displays to the same lower resolution before dragging one onto the other and then I could set my desired resolution on dummy screen and it got mirrored onto physical screen.