The only hard and fast rule is you must hit your ideal saddle height if you're to pedal in comfort and with power. Every tandem needs a triple crankset and a mountain bike-like cassette that goes from 11 or 12 teeth to 30 or more. Tandems can be reluctant climbers, so you'll appreciate a 30x30 low gear. On a tandem, it's nearly impossible to have a gear that's too big or too small. Almost all tandem frames use oversize tubing for strength and stability.
Titanium remains rare and rich.
If you need to ask the price, you can't afford it. For casual riding, shop in the cruiser category. Keep in mind this list isn't anywhere near all-inclusive. We've picked good tandems that showcase features or characteristics you should look for while shopping. Look them over, figure out what works for you, grab your best friend and go for a ride.