If the Ones are being used as surround speakers with the Arc, the only way to make them play the same audio as the Arc is to remove them as surrounds and create a stereo pair with the Ones. Then you have to group Ones with the Arc in the Sonos app. But you will probably experience a slight audio delay from the Ones when playing TV audio.

NOTE: By removing the Ones as surrounds, you will lose any Trueplay tuning you had with the Arc/Ones setup. And if you want the Ones to act a surround speakers again, you have to separate the Ones as a stereo pair and add them as surrounds to the Arc again in the Sonos app.

It has nothing to do with Sonos’ “unending quest to minimize system flexibility”.  Setting up surrounds requires reconfiguring the radios to be a direct private 5 GHz connection to the main component.  This reconfiguration takes a little bit of handshaking, so it’s not an instantaneous switch from one to the other. Which is why setting up surrounds takes a beat or two.

Besides, if you did set them up to duplicate the Arc, there would be a lag in the Ones due to buffering, causing a distinct echo effect.  

The request in this case wouldn’t really require creating a separate pair though, since the speakers will still always play the same audio as the Arc they are connected to.  This really is just about having the option to play the same channels of audio as the front speakers when the sources is stereo.

Danny, that would be a handy option, like the Music Full/Ambient option.

Don’t reconfigure anything, just flip a controller setting to send the L/R stereo signals over the 5 GHz link instead of the surround audio.

You should be hearing some ambient audio from the surround speakers when playing a stereo source. Have a look at this article and try to set up your dad's TV audio setting to passthrough, if possible.

https://support.sonos.com/en/article/low-audio-from-surround-speakers-when-watching-tv

… additionally, you can find more info here:

This would be a great feature - most of the time I would prefer the playbar/arc with surrounds to work as stereo sound.

I was searching Sonos questions on the Internet and came across your post from a year ago. I don’t know if you’ll ever see this reply, but you did a great job in explaining the surrounds using the private 5gHz connection. I remember Sonos explaining this to me years ago but I couldn’t find where that was documented and explained. And you raise a great point about the possible echo effect when configuring the surrounds as sited above.