User`s manual

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6. Use both extended and mirrored displays
To work with both extended and mirrored displays, uncheck the box
of mirror displays as the circle mark below, then in the Arrangement
pane hold down both keys cmd and alt and drag the display
you wish to overlay the primary display. Repeat this with each display
you wish to mirror, those untouched will remain extending.
Both of the additional monitors are extended
One extended and the other mirrored
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