Double exposures are a pretty neat way to create striking images that instantly grab the viewer's attention. They can be quite difficult to get right in camera, however. Luckily, that's why we have Photoshop.
The beauty of Photoshop in this case is that you can select any two images to merge, then adjust the blending and add extra effects to really polish the final output. When doing this, one of the key steps is to make sure that you send the whites in your overlaid image to true white; don't be afraid to blow out a ton of the highlights. When you change the blending mode, this will automatically strip out those bits, saving a lot of time from having to make an otherwise very complex selection. Also, don't be afraid to go back to the gradient fill layer you created at the outset when all is said and done and adjust the colors and spread; because this layer essentially frames the subject, you may find that you want to adjust it to best complement the other colors once you've completed toning the two overlaid images. I'd love to see your results, so be sure to share them in the comments!