Jeffy, honesty is the best policy in my book. Call your 2 night renter and tell him of your situation and see if you can work out a solution with him. He might be willing to move to one of your other properties, especially if you make it worth his while. You can offer a discounted rate...a free night...a gift certificate to a nice restaurant. Or he may be a good guy and help you out without asking for anything from you.
Do you usually accept two night rentals? In our market, we take them only in January-February (slow winter season in the Smokies) or with last minute rentals (a week or less out). By taking a 2 night stay this far in advance, you kind of sealed your fate. I've had to learn from some bad choices on my part too and this may be yours. Usually when something like this happens, I revise my rental policies so I can avoid making the same mistake twice and having something in writing when guests challenge us on our policies. Hope your renter is understanding and helps you out of this pinch. Good luck!
Take a look at what you will earn on the long rental compared to the 2-day -- it might even be worth your while to give him a completely free stay at your other rental. But if he insists on the original agreement, you have a contract and I think you should honor it. It's an expensive lesson, to be sure. Hiring a lawyer to defend yourself will be MUCH more expensive, as will dealing with a bad review on Homeaway.
Like AmyG, I avoid this by not accepting short rentals until much closer to the check in time. People who stay for long periods usually plan much further ahead. In my region it's generally safe to open up to shorter rentals only a month or two in advance. Your rental area may be different, of course.