Well, they never give you too much information about the 2nd day free program because it's a perk they want the freedom to cancel at any time, but both times we've used it, you need to go to a little window just inside the admission turnstiles, and get your picture taken... they then give you a plastic admission pass with your picture and name on it for the next day (actually, often it's longer than one day free, depending on what promo they're running) So the pass is non-transferable.
However, I must say the people working the turnstiles are totally uninterested in scrutinizing your pass. So you could probably get away with using someone else's pass.