It depends. If the coupon codes you're applying are for different products (e.g. one code for 33% off photo books and one code for $5 off ornaments), then the shopping cart will accept multiple coupon codes.
However, if the coupon codes are for the same product (e.g. one code for 15% off photo gifts and one code for $5 off ornaments, which are classified as photo gifts) and/or stacked on top of a site-wide promo (e.g. one code for 15% off prints and one code for $10 off a $50 purchase), the shopping cart will not accept multiple coupon codes.