The Single or Dual Wallpaper selection applies only when your monitors are connected in extended mode. In Clone Mode although you may have multiple monitors connected, but as the same desktop would be visible on all the monitors, hence this topic will be applicable only when your monitors are connected in extended mode.

Enable or Disable Automatic Wallpaper Changing

In order to set a single wallpaper, you can use windows wallpaper control panel and in order to use dual wallpapers you can use Dual Monitor Software. In case you would like to have a single wallpaper image and would still like to use the Dual Monitor Software, you can switch off the functionality of the software to manage dual wallpaper images automatically by unchecking the Wallpaper Controller check box in the Monitor Background tab.