Maybe the cards could have holes in them, in different places, like punchcards. And if you place a card on top of another card, in way that shows both its attack values, you get to use both numbers!

(Side note: Rude, I can't remember if you tried Age of Conan or not. It just went free to play, and I've always been kinda intrigued. Any thoughts?) (Side note 2: picked up Recettear for $5 in the Steam Sale!)