Yep guess I was confused when they said isn't supported for using different wallpapers on different monitors.
Answer merge two images together that fits your dimensions.
In my case (2) 1680x1050 LCD monitors = 3360x1050 for image size.
Using Nemo right click and set as wallpaper. Then go into Cinnamon settings>Backgrounds
Click at bottom advanced options and for picture aspect select Spanned!
Worked even when the Mars pano shot wasn't in the default cinnamon wallpapers location.
Might not be news to you. But was fer me and made me smile to have one image span across my monitors.