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.
Additionally, if you have photo credits in your account, those credits will automatically be applied first for products in your cart. If you desire to also use a photo promo code in the same cart with your photo credit.
How can I redeem a current Walgreens.com Photo Promotion?
Where can I find current Walgreens Photo Promotions?