As ajgoodfellow says, you need to use ultramon or similar.Guys any idea how you merge the desktop wallpaper when running dual monitors? Most off you dont bother, I would have thought it would look better?!
To get the wallpapers to merge when using screens of different sizes, you have to use the left and right wallpapers of the same resolution. eg. if you're using a 20" and 17" then you have to use the 1680x1050 wallpaper on the 17" as well as the 20" otherwise they won't merge properly.