This particular scenario can be achieved by using the .sf_cols divs to set your background image, You can set one background-image to the other layout div and different for the divs of the elements inside the other layout control. I have recorded a demonstrative video for you. However, if you want to drop widgets in the outer layout control, this will not be possible, because it is covered by the layout, placed inside it, and you can only drop widgets in the inside layout. Here's the video.
Hope this helps!
What I sent you was, indeed, a video, demonstrating what I meant. I'm glad that you found my response useful!