According to their website they don't sell a 2 day ticket. They sell single days, annual passes and something called a season pass that expires in December no matter when you buy it. You have to use the season pass before Memorial Day and it's blacked out Saturdays and Sundays. If you used a season pass on Saturday, did a tour on Sunday, you could then use it on Monday, as long as Monday isn't blocked. But otherwise you'd just have to buy 2 separate tickets.