The more light that hits the film, the darker it gets. That is the main thing to remember as that is what affects how the two images will look like on top of each other. Dark areas on one image (little or no exposure) will allow the other image to dominate in those areas. Highlight areas in one image will block information in those areas from the other image.

Her skin tones are just above middle gray, so it allows for the information from the other exposure to come thru...but not as much as in the darker background.

Another important thing is how you are going to print it. If you are using a double exposure, two sandwiched negatives, or two seperate negatives exposed seperately onto the photo paper, the results will differ. In the last method, it will be the shadow areas of one image that will block the information from the second image.

So how you plan on printing the image will determine how you go about making the negative(s).