Sammy,

Going tandem is a good way to get them started. I did exactly that with my wife and (then) 10 year old daughter. I started by renting one for them on weekends for organized rides, and ended up finding a late 80's/early 90's Univega mtb tandem relatively cheap on CL for $200.

I put another $100 into it what with new grips, saddles, brake bads, and slicks. But for $300 they had a ball for a whole year on it, and racked up about 7-8 metric centuries. On one organized ride, they managed to spin that old tank up to 53+mph on a downhill....

My girl has since graduated to her first road bike, and the tandem sits in the garage gathering dust. Too bad you're not closer - I'd make you a deal...

Anyway good advice up above - don't skimp on brake components, and make sure the wheels and tires are good. Hands=on shifting is a good thing - mine has twist-grip. Other than that, they are pretty much the same as wrenching on regular bikes, except the extra drive side crank and the extra chain.

As T-Mar said, cooperation between the two riders is critical, and it is a learned thing.