Sometimes photographers over-do it. I know I do. We try to fill the background with abstract interesting lines. We put stuff in front of our lens to get dreamy, cool effects. We increase the saturation a bit too much or add just a tad bit too much contrast. With double exposure photography it's the opposite. The goal in the end is to create a simple, beautiful fine art photograph. There's 3 things to consider in order to achieve this goal.
Subject, Simple and Subtle
Subject: You have two subjects in double exposure photography. The first subject is generally the outline or silhouette of the top half of a human-usually facing to the side in order to get their facial profile (there are always exceptions to this but in general this works the best). This is basically a portrait with a solid blown-out background and most of the human is underexposed. As you practice this you'll figure out how to light the subject in creative ways according to your style.
The second subject is the "fill" image. This is the second image you take to fill in the outline of the first image. It might be flowers, a tree line, building, mountains, etc. This image should be generally focused throughout the area that is doing the "filling". There should be a distinct texture with contrasting lines.
Simple: Your background should be simple. This means that when you shoot your silhouette portrait where you "blow out" the background, the area around the person should be free of other objects like trees, buildings, power lines, etc. The fill image is very abstract and difficult to decipher from your audience's perspective, so making sure that the background is clear of unneeded distractions is important.
Subtle: Your editing of double exposures should be quite subtle. Some specifics elements to pay close attention to are saturation, contrast, exposure, highlights and shadows. A slight "matte" effect is generally a good choice when editing double exposures as well.
The reason for editing with such subtlety is that the image is already very abstract and interesting without color and contrast added. It has a surreal, fine art feel from the beginning. If we edit a double exposure just like a regular portrait then you run the risk of taking away from the "beauty" feel and creating an almost "illustrated" feel. We want this to still be photography when we're done so instead of increasing the usual editing elements we may need to decrease them. I have a whole filmpack of presets that I personally use specifically for editing double exposures because the differentiation is so great.
Recently, I was on a test shoot with a model and another photographer and we attempted to apply my 3 double exposure principles to craft a simple double exposure photo. I shot this with the 5D Mark lll \\ 50L.
The initial portrait settings were 1/250 ƒ2.8 ISO 100.
The textured tree line settings were 1/100 ƒ4.0 ISO 100.