This effect is extremely popular nowadays and you have surely seen 2 overlapping images on covers of music albums, in magazines, and ads. Double exposure photography has a long history, rooted in old film photography techniques, and this approach is extensively used in image editing nowadays. If you are just learning how to create double exposure in Photoshop, I recommend using a dark base photo with a rather neutral background and few details. In most cases, retouchers use dark portrait pictures for the base.
As for the second photo, you should opt for a bright shot with many abstract patterns, intricate elements, captivating textures, and whimsical details. For instance, you can choose a landscape image, pictures of flowers and vegetation, forestry, trees and urban architecture for layering on top of the base photo.