One line out from PC to two Sonos speakers?

Tough one. Sonos does not make for a good computer speaker. There is at least a 1/2 second delay (up to 2 seconds depending on software settings) between signal hitting the line in, and sound coming out of the speaker. This is for Play 5 or the connect series. The newer hardware designed for home theater will be better, but this introduces other issues. This is why I went with a Bose system for my daughter's desk instead of Sonos like we have in the rest of the house. The input on the Bose cannot be shared around the house, but there is no delay to process the signal. I don't know what Sonos hardware you have, but a play base or playbar could work if you have HDMI out from your computer and your don't mind a Sonos designed for TV plugged into your computer. There are better, non-Sonos solutions out there.

Tough one. Sonos does not make for a good computer speaker. There is at least a 1/2 second delay (up to 2 seconds depending on software settings) between signal hitting the line in, and sound coming out of the speaker. This is for Play 5 or the connect series. The newer hardware designed for home theater will be better, but this introduces other issues. This is why I went with a Bose system for my daughter's desk instead of Sonos like we have in the rest of the house. The input on the Bose cannot be shared around the house, but there is no delay to process the signal. I don't know what Sonos hardware you have, but a play base or playbar could work if you have HDMI out from your computer and your don't mind a Sonos designed for TV plugged into your computer. There are better, non-Sonos solutions out there.
Incorrect. The delay in Uncompressed mode for Line In is 70ms (not 500ms). While a Sonos system does not make for ideal PC speakers, I use it very successfully with a Connect between my Mac’s output & a good quality 2.1 PC speaker system.

You could buy one or two Play:5 speakers, and connect your PC to one them via its Line In. Works fine with a single speaker, or with two as a stereo pair, or with two grouped. You can also push the audio to any other Sonos device.

I stand corrected on the input lag. 70ms is what the SONOS site states. When I tried this setup, I found the lag unacceptable. Your use may be different. Watching videos had poor lip sync to me. The number of speakers you use should not effect this, so I would try to experiment with your current play 5. If it is acceptable to you with your 5, then using other speakers should be fine. Another issue is that the smaller speakers (1, 3) do not have a line in. You need a connect or 5, or play series to get a line input. You could use a pair of play 5 in stereo...would sound good as long as you find the slight delay tolerable. Again, depends on what you use your computer for.
Yes, 70ms delay is in the range where lip sync is just detectable. I find it acceptable for casual video watching on the PC, but wouldn’t put up with it for my main TV. For audio-only applications, however — including audioconferencing — it’s not detectable.

There are many, including new Bose models that have wireless and/or bluetooth. What is also needed is a line in jack, that is also quite common.
But their wireless stability will almost certainly not be at Sonos levels, so you have decide what matters more to you.
If the Amazon Dot works well in your location, you could wire that to any pair of computer speakers and use that to play music via the PC as well if the latter has bluetooth, as well as from Spotify on your phone. And of course use it as you would use a Dot with voice commands, with its responses coming via the speakers.

I guess the 70 ms delay is the perceived problem and the reason for looking for an alternative;