Japanese artist Negi Hideyuki, uses two sketchpads to create amazing 3D artworks. Actually, he can work with one or many as long as the surfaces are perpendicular to each other or forming a 90 degrees angle to create the 3D visual effect, but most of the time he only uses two.
Colored or black and white, they are all amazing. Most of his original works are already sold out. See more of his artworks here.