Did you also post this in the Crochet Forum?
Crochetcabana.com has sewing/seaming tuts for crocheted pieces.
Rather than actual sewing you might consider crocheting them tog. How visible, or not, you want the stitches to be is a matter of personal choice. Some joins are meant to be decorative, some are strictly utilitarian. You might want to do a few tests and see which look you like the best.