Unplug both speakers from power for a couple of minutes and reboot your router. Then follow the instructions in this article to create a stereo pair:

https://support.sonos.com/en-us/article/create-a-stereo-pair

Thanks for help, but that did not work for me.

Where in the process did it “not work”? What error message did you get? 

When I try to configure a stereo pair I am getting the message: I need two compatible speakers…. One of the speakers are a few years older than the other one, but still a One SL speaker. When using Spotify I can choose one speaker at a time, but not both.

Is one of them a Play:1 and the other a One?

You can check in Settings/System where the Rooms and the associated speaker(s) are listed. 

Both marked One SL

Most peculiar. I think you need to generate a diagnostics report and contact Sonos support. But you’ll prob need to leave that till after the weekend. Let us know what they say. 

Problem solved. After resetting the speakers and re-installing them in the Sonos app it was working. Now playing in stereo. Thanks to all inputs from “nik9669a” and “GuitarSuperstar” :-)