Syncing two home pod in different rooms

To split a stereo pair use the Home app, select your HomePod pair with a long press (otherwise it does play/pause) scroll down to the gear icon at the bottom and tap that, then scroll down again to Ungroup Accessories. See Set up HomePod stereo pairing - Apple Support for details.

If you're talking about playback you can say things like "Hey Siri, stop playing in <name of room>" when you'd originally asked it to play everywhere.

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