Lets say you have two layers.  On the first(bottom) Layer 0 you have a sky, for a background.  And on the second Layer 1 you have a render of a person, with a different background.  ANd lets say you like the person of Layer 1, but would prefer the background of Layer 0.  

Both Layers would, simply, need to have 100% Opacity;  but you would then select the background, of Layer 1, and Edit > Clear it.  Then all of the pixels that were Cleared would be Transparent Pixels, and the Background of Layer 0 would then show through those pixels.