How to span single wallpaper over dual monitors?

if using gnome-shell, in tweak tool under desktop there is an option to have the image span the desktop. This makes it go across both monitors(or all).

No Nitrogen Necessary

Unity also has this same option as shown below:

First you need a wallpaper large enough for both monitors. You can either make one with the gimp or down load one. Alternately you can use a separate image for each monitor.

You can then install a handy little application, nitrogen

sudo apt-get install nitrogen

You then run nitrogen with the path to the directory with your picture(s)

nitrogen ~/Pictures

And select the image. At the bottom of nitrogen, select "automatic" and "Full screen" as options. alternately you can use nitrogen to set a separate image on each screen, up to you (sort of depends on your background images).

Nitrogen answer works, but another tweak is necessary. Run Advanced Settings in Gnome Tweak Tool, go to Desktop section and switch Have file manager handle the desktop off. Then wallpapers set via Nitrogen will be displayed.

If you do not have the Gnome Tweak Tool, you can use this command instead. It's an easier solution, too:

gsettings set org.gnome.desktop.background show-desktop-icons false

However there is strong disadvantage. All icons disappear from the desktop and the context menu can't be invoked. Unless you need icons on desktop this solution may be useful for you.