1. Tandems are Not Cheap. Buying a cheap one, either new from a "discount" bike warehouse or second hand from eBay is a recipe for having to shell out a lot more in repairs, upgrades and accessories. Budget for at least £500 for even a cheap second hand one.
2. Don't be tempted to buy a "vintage" tandem unless that's exactly what you're after. The parts will be hard to get and it'll be a pig to ride.
3. Avoid downtube shifters if you're a novice team. You'll want to keep your hands on the 'bars as much as possible.
4. Look at the wheels VERY closely when buying a tandem. 36 on the front and 40 on the rear is a minimum. Tandems break rear spokes like crazy if the wheel's not up to scratch.
5. Unless you're getting an exotic confection like our new Santana, a drag brake is a must. Descending with only standard solo brakes can be hairy.
6. Never buy a tandem with a disk brake on the front.
7. Erm...
That's all I can think of right now. I'm sure a more experienced
tandemista will be along inna minute