For seams on finished knitting and especially for an afghan should be reversible, I'd make a virtue of necessity and make the seam part of the blanket or throw. Ypu can crochet the scarves together using one of the yarns as an accent. Or you can try seaming with a
baseball stitch that abuts the pieces rather than creating a bulky seam. (The seaming is late in this video. Don't give up hope.)