SUMMARY
It would be awesome if wallpapers could be set to span across multiple desktops, like having the left part on the left screen and the right part of the wallpaper on the right screen. This should support not only pictures, but every wallpaper engine installed.

STEPS TO REPRODUCE
1. Select a wallpaper on a multiple monitor setup.

OBSERVED RESULT
It is only set for one screen.

EXPECTED RESULT
It should also be possible to select one wallpaper across all screens.

ADDITIONAL INFORMATION
Internally, it might be possible to show the wallpaper engine a single virtual monitor with the resolution of all monitors added. Then split the picture that the engine returns and set it to the monitors afterwards.
Example for 2 Full-HD monitors next to each other:
1. Create a virtual monitor for the wallpaper engine with the resolution of (1920*2, 1080)
2. Let the wallpaper engine render a wallpaper image in that resolution
3. Split the wallpaper into two halves in the middle and set those two images to each of the screens.