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.
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.
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).
My cleanup wasn't perfect in this slide, but it still turned out pretty well.
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.
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.