Yes, many patterns say to use the yarn doubled. That makes a thicker yarn, so your work is chunkier and works up faster. You CANNOT use a heavier weight yarn since that is too bulky and heavy. By using two yarns, they will lay side by side in the stitch, making it look wider (thicker). But there is not the bulk of a thick yarn, or the thickness of the fabric. So you get larger stitches without having all that extra weight from a thick yarn, The cardigan is not as heavy or as thick if you had used a thicker yarn – and less cost. All this is important!