Any Sonos speakers which run the same OS can be grouped. 

If you’re wanting to stereo pair with a Sonos One, you may have more difficulty, only another Sonos One speaker can be stereo paired with a Sonos One speaker. 

It depends on which two speakers you currently have. Most Sonos speakers can be grouped together or used in stereo pairs, but compatibility for features like surround sound or pairing as rears depends on the model. If you let us know the exact models you're using now, I can help you pick the best additions.

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