What kind of borders or selvage sts are on the dishcloths? Garter, moss?
ETA: If garter, there's seaming info (w/pics) here. This site has more chapters on horiz seaming, as well.
You'll have to decide how 'invisible' you want the join and if you want a seam on the back...or not. The nice thing about crochet (which can be learned quite easily) is that it works up quickly and can be 'unzipped' if you're not getting the look you want or need to re-adjust for variations in the sides of the sqs. (Bit more arduous taking out sewn stitches.)
Are you planning to seam in matching yarn or have the joins be a contrast?