You would need to unbond them from the Beam, so that they can operate either as separate Sonos rooms, or as a stereo pair.
Easiest way is to perhaps get another pair of Sonos speakers for the Kitchen - saves all that to-ing and fro-ing
Sonos does not do this well. As you can see in the Sonos app, all speakers are in a “room” in Sonos. If you want to use the speakers elsewhere and not have the Beam playing you need to set them up as a separate stereo pair, as explained above. This is a chore, caused by the very different way surrounds are bonded to the Sonos system, compared to them beying in separate “rooms”. You will also need to do Trueplay again after every change.