Read the blog entry about it.
Here's the README for the command-line tool.
The GUI tool is not quite finished, but at least it's functional. You can drag and drop images from, say, Nautilus, and then press the "Set as Wallpaper" button.
You can check out the latest version from my Subversion repository:
svn co https://mg.pov.lt/dualpaper/svn dualpaper
Or you can browse the repository with a Web browser.
dualpaper was written by Marius Gedminas. It is released under the terms of the GNU GPL.