Suddenly I have my two displays mirroring each other - I can't make this stop

As leroydouglas said, this can be triggered quickly (and also, as in this case accidentally) with Command-F1 or Command-Brightness down.

Also: in System Preferences->Displays, click Display Settings...

And then select one display as the main display, and the other as extended display:

abridge683 wrote:

I have a MacPro driving two monitors, a 4K monitor and a smaller one. They are connected to my MacPro via DisplayPort.

I was attempting to start the Zoom function for the 4K when suddenly my large 4K display began mirroring the smaller display. I cannot figure out how to turn the mirroring function off. It's like something in Accessibility has turned this on.

I'd like to return to things as they were! I need the two monitors to be separate!

Thanks for any help you can provide.

Command-BrightnessDown

toggles the setting of : [√] Mirror Displays

Try using Extended Desktop Mode instead of Mirror Display Mode. Mirror display also mirrors the display resolution. Extended desktop mode allows you to select the native resolution of the external display.

Use external monitors with your Mac: https://support.apple.com/en-us/HT202351