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The theorized setup you wish to have is possible. Here is some information that may be helpful:
Set up some displays for mirroring and others for an extended desktop
If your computer is connected to more than two displays, you can specify that some are mirrored displays and others show the extended desktop. For example, if you have three displays, you can have two displays showing the same information and the third display showing the extended desktop.
1. Set up all the displays as an extended desktop.
2. Option-drag a display icon onto another display icon to mirror those two displays.
Furthermore, you may find this resource helpful as well: