This version of a double exposure works well with a subject surrounded by a white background. If your subject was not originally photographed in front of a white background, you can fill it with white in Photoshop.
Original Source #1
Original Source #2
Final Version (color)
Final Version (black and white)
Step 1: Open your original source file and select the background with the Quick Selection Tool.
Step 2: Click on Select and Mask from the Options bar and refine the edges. Click "ok" when the edges look cleaner.
Step 3: Pull down the Edit menu to Fill and select White under "Contents".
Step 4: Open your second source file, select all (Ctrl A), copy it to the clipboard (Ctrl C), and paste it (Ctrl V) into the image. Use the Transform command (Ctrl T) to resize it so that it completely covers the background subject.
Step 5: Pull down the Layer Blend Mode menu in the Layers palette to Screen.
Step 6: Use the Move tool in the Tool Bar to move the top source around the image, as well as the Transform command (Ctrl T) to do any additional resizing or reorientation of the second source image.
Step 7: (Optional) Use the Adjustment Layer menu to do further editing.