The book I had in mind is called "Shadow Quilts". But I think you want two different blocks which create a third "block" when combined, and I have two books with patterns like that: "Pairing Up, 2 block quilts" by Nancy Mahoney (which is great!), and also "Creative Two-block Quilts" by Trice Boerens. As Jan in Va already said, you can usually accomplish this kind of third "block" by using a block with diagonally pieced corners. One block which is simple and works well like this is the Snowball block. Have fun choosing a design!