You cannot stereo pair (or use as surrounds) anything but like models (Ones with Ones, Play:1’s with Play:1s).  Rumor has it that there was an iOS app that allowed you to work around this limit, but I’m not sure what the status of the app is today (not an iOS person). 

Thank you for your prompt reply....

my reading was that they were compatible as "from the same family".  I shall not spend more time on this ......

You can group them, so they both play the same music, in sync. 

Thank you nik9669a....

I've got them playing simultaneously but was trying to create a stereo sound between both but with no luck. Happy with sound anyway but very curious on how it would sound in stereo. 

Imho Sonos knows best, if any pair of different speakers would fit well as a stereo pair. And though the Play1 really sounds quite similar to a Sonos One(SL) there may be other reasons of different hardware that speak against such a setup. 
If you use any 3rd party software to configure a setup that isn’t supported by Sonos you will be out of any support help. 
Also damages caused by that will not be handled by warranty. 
But those who love the risky side of live can try that iOS app I will not mention by name. 

Thank you, Schlumf..... I'm sure there's a "workaround" but I won't risk it.... got the two speakers to my left & right..... I almost have "stereo" sound..... and can still chill  

Good decision imo…