Can I mirror my iPhone to multiple Apple TVs
How do I airplay mirror to two or more TVs at once?
iPhone 7 Plus, iOS 11.0.3
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You don't. You can only AirPlay to a single receiver at a time.
If you need to send the image to more than one TV at a time, you can use an HDMI splitter and send the video signal from the Apple TV out to more TVs over HDMI cables.