In these sketches from the San Juan Islands the foreground shape and the background are kept separate through manipulation of color, value, composition and wetness.
Along the edge that marks the shared boundary of the shapes the variables have been adjusted for maximum differentiation. First of all, the edge where they meet is hard. It's also no coincidence that the value of the background headland is darker where it is overlapped by the foreground promontory.
In both of the images above, the value range has been compressed somewhat in the background to suggest distance. The difference between the dark and the light is narrowed deliberately to approximate what we are accustomed to seeing. In the scene below, the background shape is meant to appear much more distant, so the range from light to dark is eliminated altogether.
Even in the photo it appears to be in the same plane as the cliff on the right. Go down the list of variables to consider what you can adjust to make the separation more obvious.
Here's another possibility: