Hi. I actually have this Pacific Dualie tandem bought from Amazon.com (offers free shipping!).

http://www.amazon.com/Pacific-Dualie...0728161&sr=1-3

It has a thick, stiff oval steel frame and it doesn't have any noticeable flex (that noodlely feeling you can get on old rental tandems) but it is heavy if you're used to racing single road bikes. I find it ideal for neighborhood tooling around with the wife or kids.