It depends too much on how conversant you are with your machine and if you could find a command that would do that. Then you get to find a way to tie it for the Fn button.
For now I'd look at other ways such as a cheap solution we used for a trade show booth. We wanted such so we used a VGA or HDMI switch box. Done.
Bob
Is it possible to set up a function key that will toggle an external display on and off?

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