Have you considered using a crochet joining? You could work a single crochet edge on the blanket, then crochet that into the edge (if you crochet). If you don’t crochet, or prefer to sew them together, use a wooden needle and either whip-stitch or ladder-stitch (the stitches on the right side are at right angles to the seam with the “slant” on the verso) would be fast, strong and easy. If your yarn is heavy, you could split it down and use just one or two ply.