Can you tell it's two separate images edited into one?
Probably. Composition is awkward in this crop, etc.
But I shall work on the idea and I am sharing it with you. I intend to make watercolor double exposures. The problem is...I cant paint faces well. Perhaps silhouettes I'll try. In Photography, in Double Exposure, the classical case is to use a human or animal silhouette or an underexposed portrait and to blend it with some natural landscape or so...
Nice Idea, I would not notice if you didn't wrote it :D About the faces, try some "cheating" . Take a photography of a face, make it b/w max contrast. Separate the most dark and the most light areas and paint them in photoshop, mirror horizontally, print it. Take a soft pencils and draw over these areas. Then put the sheet over your drawing paper, find something to press and transfer some traces of the pencils and use them do draw over them with perfect proportions and some light referеnce :)
Ah, a cheat sheet v0.2...