I use Ultramon and see the exact same problem. My setup is as follows:
2 screens, 1 dual-screen wallpaper, fences on my primary screen only.
my secondary screen gets an copy of the background from the primary screen placed on top of the actual wallpaper (I can see this as my secondary screen is widescreen and hence I can see a stribe of the real wallpaper in the right side of the secondary screen).
