A bit confusing at first, but great results (pictures)
Union for iOS lets you blend photos and make double exposure shots, but it takes some work.
Double exposure
While the app blends images and other cool things, most people will want to try their hand at a double exposure shot. I was somewhat successful, but it took some trial and error.
Managing layers
The main interface of Union lets you touch each layer of your project to make edits. When you're finished, you can flatten it into one layer so it's ready to save and share.
Masking layers
The Mask layer lets you clean up the Foreground layer so the images look more natural together. You can touch the eye icon to show where you've erased parts of the photo (and find the parts you missed).
Double exposure cleaned up
My cleanup wasn't perfect in this slide, but it still turned out pretty well.
Switching transparencies
To help you get the right blend between images you can touch the icon in the top center. From there, experiment with the different types to get the best look.
Sharing
When you're finished, you can save it to your camera roll, open in Instagram and other photo apps, or you can use the iOS 7 share features to sent to Facebook or Twitter.