A UV Layout can only have one image, although you can tile and animate the image. Since a layout is a bunch of arranged UV Maps, and a UV Map maps many mesh faces, a face can therfore only have one UV Texture image, and the UV coordinates for that face must fit entirely on the image. If you want a face to have multiple images, split the face into parts, and assign each part its own image.
Thats from the same page… and on the previous page:
Merging UV Maps
Each time you unwrap, Blender creates a set of UV Coordinates shown in the UV/Image Editor window. So, if you unwrap 2 sides of a cube, and then 3 sides, and then the other remaining side of the cube, there will be 3 sets of UV Coordinates or UV Maps. You would unwrap this way if you wanted to use three different images for each of those maps. However, if you change your mind, simply select all the faces (for example, all 6), and re-use the Archimapper, the UV/Image Editor window’s UVs->Unwrap menu item. The maps will be recomputed to be non-overlapping
I dont know if this will help at all, but its worth a read:
Might help explain the use two different textures such as in the picture you posted.