Windows 10 dual monitor wallpapers post Creator Update (v1703)

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.