Hi Janno, you can in fact do this now, as long as you are a registered user. To achieve this you need to use the green photo basket at the bottom of the editing area. See our Photo Basket help for more details.
We can layer the photos, and use the fade option with that, but fading isn’t what I want to do with layering photos. I would like to be able to erase part of one photo to show the underneath of the other. Example: Make it appear that someone is flying in the sky.