Assuming you have added the One SL’s to the same Sonos Household, see if this link assists…

https://support.sonos.com/s/article/1066

Or, if you prefer for both speakers to be mono, you could:

https://support.sonos.com/s/article/3391

Hi I’ve followed the instructions in the App when I get to ‘set up stereo pair’ that option isn’t highlighted snd I can’t select it

I’m just wondering if you may have the older Play: 1 (rather than a Sonos One) and a Sonos One SL? - just checking, because if that’s the case, you cannot stereo pair those?

If that is not the case and it’s definitely a Sonos One, then I would also check to see what firmware version the speakers are running in ‘Settings/System/About My Sonos’ and maybe check the Sonos App again for any device updates.

They are definitely both Sonos one SL as I’ve checked receipts for both purchases- bought the first one a few months ago from Richer sounds and simply ordered a repeat purchase a few days ago. That did cross my mind though. 

Thanks for this.

I’ll spend tomorrow morning figuring it out and also double checking that the speakers are indeed compatible. 
cheers 

The ‘about my sonos’ should confirm they are the same type of speaker - I think checking for updates and ensuring both are running the exact same firmware might resolve the matter.

I’ll try that first thing. 
cheers