I need to talk to you about this, I have only don't a couple with my Mark III. I love what you have been doing with yours. I haven't tried any on me only on plants. I had a tutorial but ended up going to google to research more. Haven't got anything near as good as you are getting. I need to pick your brain.
Barbara, look up Sara K. Byrne Photography, click on her blog, scroll down a bit, she gives a video turtorial on how it's done in camera. You might have the Canon camera that has a "live view" to know where you are placing the second exposure which is pretty cool, my Nikon I have to wing it. Hope this helps, can't wait to see yours... right now I'm wishing I had the Canon with those cool features.Janice