I am using dual monitor for my work, currently running mint 18, Now i have question can i use different wallpaper on my both monitor in loki ?
Have you tried this application? :) Multi Monitor Tweaks
It seems to be a tutorial for exactly the thing you want. ;)
Nitrogen doesn't seem to work in elementary OS for some reason. Or should i say, the app itself works and you can set the different wallpapers, but then the configuration made in nitrogen doesn't work.
i think a way to override the desktop settings in "system settings" to allow nitrogen to set the wallpapers is what is needed. unfortunately im looking for that solution myself...so far to no avail.