You can actually apply one material per surface --- an object can have a different material on each surface.
For textures, however, you are limited to one texture per object. If you want different textures on different "walls" of an object, you can separate the surfaces into two different objects. i.e., select one "wall" and perform a "Surface -> Cut away object", and it puts the selected surface(s) into their own object.
Hope that helps
|