Box (EX9.Geometry) has no substets, as you can see when you connect Count (Ex9.Geometry Mesh) to it. Its just one Subset and Texture Coordinates are wrapped around that one.
You could change that by changing the Texture Coordinates by your self using Mesh (EX9.Geometry Split),VertexBuffer (EX9.Geometry Split) and VertexBuffer (EX9.Geometry Join), there you see the Texture Coordinates.
But i think its easier to model your own cube and assign subsets to it or why dont you just use two quads?
You can assign Subsets to a mesh by texturizing each face that should be a subset in a 3d Program.