Windows 10 dual monitor wallpapers post Creator Update (v1703)
Solution 1:
In Creators Update, you can open the classic Desktop Background control page using this command:
shell:::{ED834ED6-4B5A-4bfe-8F11-A626DCB6A921} -Microsoft.Personalization\pageWallpaper
Compared to the modern Settings, the classic page is easier for setting up multi-monitor wallpapers.
Solution 2:
There's an even easier way built into the update. Go into the Personalize menu and select Picture as the background, then right click on an image and it will let you select which monitor to use it on.