I recently was let on to a relatively new feature in the Canon 5D series, double exposure in camera! I had experimented a bit with the traditional double exposure format through film but when I learned you could do it digitally, allowing you to troubleshoot and check IN CAMERA what progress you were making, I naturally flipped out.
Basically, you need to slightly overexpose one image and underexpose the image you'd like to overlay it with. I chose my friend Lyn to model for the original image and then as an overlay, underexposed a row of colored seamless. I also shifted the seamless out of focus to get a more hazy touch to the addition. Below are images 1 & 2 as well as the finished composite. Certainly looking forward to playing with this feature some more in the future!