Hm, then if you want them to get $5 off for any 2 shirt designs, you would need to go into Seller Admin > Cart Discounts & Gift Coupons and set up a Discount for All Cart Items taking a flat $5 off the Item Total when Min. Items in Cart = 2 and Min. Item Total = 40.00. If you want buyers to enter a code to claim this discount, you can put that in the Discount Code field; otherwise, leave the Discount Code blank to have the discount applied automatically whenever the Min. order thresholds are met.