don't buy cheap
it's better to buy a quality used tandem than cheaper newer one. tandems put 2x the stress on the components and i prefer not to put myself and my partner at risk. my thought s are to buy a quality used tandem w/26" twheels (that way you can use either skinny or fat tires). one last thought, when you buy a tandem online don't forget about the shipping. it can be as much as $300 so you need to figure it in total price. i agree with the "renting first idea" i've seen some smokin' deals on quality used tandems on e-bay just don't forget to add the shipping cost.
i have 3 tandems, it's a great experience with the right partner