Jeff,
You do glue your two layers together before you sew don't you? I cut my belt blanks with an inexpensive strap cutter from Tandy. Never have a problem. 8/9 oz. top, 3/4 oz bottom/inside cut exactly the same width, glue them with contact cement, sand the edges, burnish edges, then sew.
I just don't think the belt blanks are an economic solution if you intend to make very many belts.
ferg