Well, I can tell you that I was really happy with what developed on these double exposured pictures I made from two very different weekends. One weekend was in New York and the other was at Second Beach in Newport. This one above is one of my favorites. I took 2 exposures of 2 different seagulls. The small one looks see-through, but the larger one is more prominent. I don't know the mechanics of why they are both not see through. I just know that I like the way it looks!
Below is another cool one of some seagulls.