Snapseed has added Double Exposure and two other tools in the 2.17 update which is beginning to hit devices starting today. Having said that, as with most app updates it may take some time before everyone receives it, so it could be days before it hits all users. The two other tools that are included in today's new update are the Expand tool, and the Face Pose tool. Double Exposure should be pretty self explanatory, and will let you combine two photos into one and by blending them together it will give you a unique looking image. As part of the blend you can also increase or decrease the opacity of the image that sits on top so you can decide how much of the background image you want to see under it. There are also six different blending modes that you can apply which includes a default setting, as well as lighten, darken, add, subtract, and overlay modes.
With Face Pose, the aim here is to allow users to add corrections to photos that have faces in them. So for example, if you have a picture that you took of other people, or if you have a photo that you've taken of yourself with your face in it, you can correct certain elements of the photo by adjusting the focal length, adding a smile to your face, and by changing your pupil size. You can also adjust the direction of your face to slightly change where you're looking, though the direction doesn't really change all that much.
The last feature, which is the Expand tool, is all about expanding your photo by adding to the outside of the canvas. This essentially extends your photo by taking various elements from the image and placing them around the outside border of your photo, which you can see in the second image below. Results from this particular tool may vary based on the image that you select, and some image types will probably look better than others as well as look more natural. This update should already be hitting most devices today so if you're a user of Snapseed you can check your my apps list in the Play Store and see if it's available to update.