Santana turns out a titanium frame, if you're into permanent damage to your credit rating.
A good buy in a reasonably lightweight tandem is the Cannondale, which has an aluminum frame. My wife and I have been tooling around on one for the past couple of years. Reasonably light (around 35 pounds for the whole bike, which is slim for a tandem), reasonably stiff, plenty of room for the stoker. New 'dales can be hard to find at times - they make them in batches, so if a shop doesn't stock one, it may take some time to order.
I don't recommend fitting standard bicycle wheels or brakes to a tandem. Typically, tandem wheels are 40-44 spokes, hubs are substantially beefier, and the brakes and rims are correspondingly heavier. The tandem subjects the wheels and brakes to twice the normal load of a single rider bike, and that's an invitation to catastrophic brake failure if you're using single rider components. Never a good thing, and possibly fatal.