I think you have the right idea, but not a fan of that particular switch. I don’t think it handles enough watts and doesn’t do impedance matching or volume control.  Are you specifically looking to get something in-wall?

One thing to consider, if you’re ok with the the kitchen always being on, you only wish to turn off the outside, you could get a volume control on the outside and leave it at zero volume for off.

Here are a few options to look at:

https://www.amazon.co.uk/PVCD15-Wall-Channel-Stereo-Control/dp/B007A99ICA/ref=sr_1_4?crid=14VGSFGS95A0W&dchild=1&keywords=in+wall+volume+control&qid=1589976838&s=electronics&sprefix=in+wall+vol%2Celectronics%2C265&sr=1-4

https://www.amazon.co.uk/PyleHome-PVC1-Mount-Rotary-Control/dp/B000V52CP0/ref=sr_1_3?crid=14VGSFGS95A0W&dchild=1&keywords=in+wall+volume+control&qid=1589976989&s=electronics&sprefix=in+wall+vol%2Celectronics%2C265&sr=1-3

https://www.crutchfield.com/p_190SSVC2/Niles-SSVC-2.html?tp=87314

You sir, are a genius! That’s a great idea! In answer to your question, would ideally have something in wall - I would much prefer to have control over both volumes so I think you first link would be the best idea. Any idea how one would wire it? 

Outside speakers and kitchen left and right wires into the above volume control then other wires running from that to the amp?

Many thanks again! 

OR alternatively, use this and set it up like this picture I just drew? ? But I would then lose wireless control through the Sonos app over the speaker volume as the speakers aren’t connected to the Left L/R and Right L/R ports of the amp? Thanks again 

Thanks for this question and the response. It’s inspired me to think about my two-room set up requirements. 

I’m thinking of wiring the main room speakers directly to the Amp, then wiring a switch and/or volume control also to the Amp’s binding posts. I would then wire the second pair of speakers to the in-wall switch - see my sketch above. I’m comfortable with not being able to control the volume of the main speakers, other than through the app.

I have the Sonance speakers already for the main room and I’m looking to purchase a smaller, compatible set of speakers (likely the same brand and also 8 Ohms) for the adjacent room. My questions are; a) Can anyone spot a flaw in my thinking? b) Anything I need to consider regarding the specification of the switch itself, eg. power handling, impedance matching, etc? c) Any Trueplay implications?

Thanks in advance everyone.

I think that plan would work.  I don’t think  you’ll need to worry about impedance matching if both pairs of speakers are 8 ohms.  It does need to be able to handle a decent amount of power near what the amp can deliver.  As far as Trueplay goes, you won’t be able to use it for this setup.