Are you using NVIDIA Control Panel?
Open it and go to "Setup Multiple Displays" under category "Display".
From here, you can configure the monitors as "DualView", independant from eachother.
This is what I use, and I can see the taskbar on my main 22 inch and not on my 18 in. But you can still drag windows and shit over there. I like it because then everytime I open certain programs, it automatically opens them exactly where I wan't them on each monitor.
But I think you want "As one large horizontal desktop", where the task bar goes all the way across both screens. Since you only have ONE background, and ONE desktop, just on 2 displays, you can't have 2 desktop background running at once. That would just be crazy! Lol.
I think you should just take the 2 seperate pics and cut them together in PShop as one wide picture.
Hope that helps!