Mirror some displays but not others I have my Macbook Pro 13" (2017) connected to two external 27" monitors. 

I'm going to be getting a new iPad Pro today and intend to use it with the new sidecar functionality. My goal is to get rid of my 27" monitors and roll with the Macbook screen + sidecar. But first, I want to make sure everything is working okay with my daily workflow. 

So, what I am hoping to do is mirror my Mac's screen to one of the 27" monitors, and the extended sidecar screen to the other 27" monitor. 

I found these instructions, but they reference using the Alt key to accomplish mirroring some displays but not others. The 2017 MacBook has no Alt key. Are these instructions out of date, or am I just missing something obvious?
The Alt key is a Windows term for the alternate function key, in this case macOS calls this the option key, with the symbol ⌥.

It sits on most keyboard layouts between the control and command keys to the left of the space bar, and between the arrow keys and the command key on the right of the space bar.




  Note: On a Windows keyboard connected to a Mac it is likely the Windows key will act as option as these two keys are swapped in most layouts.


I can confirm the steps mentioned do work for mixed mirroring and extending of displays, although I am unsure if this works with Sidecar.