You could group two Amps, and waste one channel of each by leaving it idle. 

The amplifier in the Amps can’t be bridged. 

Thinking about it a second, I think you could, in a round about sort of way.  If you set up one amp, connected one of the speakers to the left or right channel, and you have one side done.  Setup the second amp and then add it as surrounds speakers for the for the first amp.  Connect the other speaker as the other channel not already covered.  You should see both amps setup as single room, and in the configuration, make sure it’s set to full stereo for music sources (this won’t work for TV sources).  You will also want to make sure that the surrounds are set to play at full volume.

This should work, but clearly not the intended design, so you have some trouble getting the two speakers to be properly balanced for volume.  And this would not work for TV sources, as one of your channels will play surround audio channel.

Alternatively, you could set both amps to play mono.  Or you could just keep the amps as separate rooms and keep them grouped all the time.

The simplest way, for the cost of two amps.

I’d spend some more time looking at wiring options to use a single Sonos Amp.

On wall or under carpet, nearly invisible.

https://smile.amazon.com/Ghost-Adhesive-Speaker-Conductor-Sewell/dp/B079NTKWS2/ref=sr_1_5

Not as invisible but when painted it really blends in nicely. I use something similar from a local store to get power to a couple Sonos, makes the spouse less aggravated. Too many turns for the above option to work well.

https://smile.amazon.com/Cable-Concealer-Wall-Raceway-Management/dp/B082X9RCZX/ref=sr_1_3

Just for completeness, it’s theoretically possible to stereo-pair two Amps (or indeed any two Sonos devices) using SoCo-CLI I1]. Each Amp would become a mono Amp, one serving the L channel, the other the R, with the same audio output from each speaker terminal on each of the Amps:

sonos <left_amp_name> pair <right_amp_name>

The only benefit of this over grouping is that your Amp pair would appear as a single room in the Sonos Apps.

However:

1. I’ve never tested this with Amps, although it does work with Connects
2. It would be completely unsupported by Sonos, and the ability to do it might disappear at any time
3. You’d waste one speaker output of each Amp

/1] https://github.com/avantrec/soco-cli#grouping-and-stereo-pairing

For amps though, wouldn’t you accomplish the same thing just by bonding one of the amps to do surround sound duty for the other, than setting music to full?

I’ve no direct experience of bonded setups, but that sounds like an equally cunning ruse :)