Double picture mounts are where two contrasting picture mounts are placed directly on top of each other usually to show a 5mm "sight line" of the bevel edged lower mount. The mounts are glued together to form a one piece mount.
Double picture mounts, that is one single mount "layered" on top of another single picture mount, perfectly cut to the same outer size but the aperture slightly larger for the top layer allows each image to have two colours "mounting" the picture. Not only does this increase the visual perception of the depth of the assembly but it gives the opportunity of creating the possibility of greater contrast and interest in the way the image is presented.
For example you could use two very light colours top and bottom to create the illusion of a window of depth, or use a pair of very dark colours to create drama. Having said that though the very best examples tend to be where the difference between top mount and bottom mount is subtle, delicate and above all enhancing to the image. Try Textured snow white and Pale Grey.. wow !