Well, the surfaces are merged into one surface. I think what you want it changing the texture coordinates. You can do that with VertexTexCoords. In the archives, there is a code for box wrapping. It can help as an example maybe:
http://www.blitzbasic.com/codearcs/codearcs.php?code=277
http://www.blitzbasic.com/codearcs/codearcs.php?code=277