Unfortunately, you'll also need a unique set of circumstances with your images to be able to make a double exposure work, so you'll need to do some preparation beforehand.
If you don't, a couple of problems will become apparent right away. You can't snap a photo from within the app (you'll need to have good images already in your photo library), and you'll want to have images on hand that have a dark foreground with a bright background. In fact, the closer you can get to a silhouette of your foreground image, the better off you'll be. Most people don't take silhouette style photos regularly (unless it's by accident), and -- without an in-app camera to snap a picture on the spot -- you'll need to plan in advance to make these double exposure projects.
Next, you can use your photo that has a dark foreground and light background as your background image (this will act as the silhouette I mentioned before). Then you can bring in your foreground image (the flowing field or sunset), and use the transparency settings at the top of the interface to blend the two images.
With the right background image and foreground image, it can come out perfectly on the first try, but it's all about planning in advance if you want to create double exposures.
Other cool tools
Union also has tools that let you change a part of an image, then replace it with something else. An adjustable Magic Wand tool lets you select a cloudy sky in a photo, for example, then replace it with a blue sky. What's cool about the Magic Wand is you can adjust the sensitivity almost like you can in the professional photo editor, Photoshop. This means you can select a part of the cloudy sky that's a specific shade a grey, then adjust the sensitivity until the selected area covers the entire cloudy portion.
The basic editing tools, with brightness, contrast, saturation, and color temperature also allow for precise adjustments. You can select an effect with your finger, then adjust a scroll wheel with your thumb at the bottom of the interface. Many photo editors just have sliders, which work well, but it's much easier to use the scroll wheel in Union.
Union is an app that gives you great results only if you prepare beforehand. I was able to make some pretty good double exposures (look at the included gallery) once I had some good photos to work with.
While there is not much Pixite can do about the specific photo requirements for a double exposure (dark foreground, light background), I think it would be a good idea to include a camera app within Union so people can quickly make some of their own.
Still, if you're willing to put in the effort, Union has great tools for creating unique looking projects.