Just sharing my experience with paired Sonos One speakers and Alexa, which might help others.
When I originally set it up the Alexa command would be recognised on both speakers. Most of the time only one speaker would respond and it did this in an alternating fashion L,R,L,R,etc. On more advanced features like using skills, then I found it very buggy with both speakers becoming out of sync and asking/responding to different things.
The simple solution......
Just mute the mic on one of the speakers. Alexa will only operate on one speaker, but when you play music it will still use both speakers.
Don't try to de-register one of the speakers from Alexa. I tried this and it did'nt disable one of the speakers 100% from Alexa command.
Hope this helps..