So they are performing Surrounds duty. In this configuration only the Right surround speaker is Alexa-enabled.
If I remember correctly from these forums, there were occurrences where both speakers picked up a command but then answered slightly out of sync, or both would increase the volume. Sonos’s solution was to disable voice control to the left surround.
Moderation edit:
With the release of 14.8 Voice Service Behavior for Home Theater Setups has changed:
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When a home theater room has Sonos Voice Control or Amazon Alexa registered to rear surround speakers, the microphones on those/both surrounds will remain enabled.
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The microphone on the voice-enabled home theater product will remain enabled and will respond to voice commands.
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If Sonos Voice is setup on a speaker that's then added as a surround, SVC will remain on that surround speaker.