If the wallpapers are appearing fine in the preview section in Superpaper app, there is no need to do anything else. Otherwise, you can tweak the diagonal size of the display, bezel size ...

Here's how you do it. Set a Wallpaper in the Settings App on Windows 10 To change desktop backgrounds individually for each ...

However, when you use a dual monitor setup, it is quite difficult to use a single wallpaper on both of the monitors. The main problem occurs when two monitors have two different resolutions.