To get started with search, recommend look at this one on
www.amazon.com
Priced at $331 with free shipping:
http://www.amazon.com/Pacific-Dualie-Bicycle-26-Inch-18-Inch/dp/B00AWNI232/ref=sr_1_4?s=sporting-goods&ie=UTF8&qid=1363972245&sr=1-4&keywords=tandem+bike
Then look on ebay for about the same bike, but with the aluminum frame at about the same price (2 or 3 brandnames). The AL framed one is a better bike. They have a buy it now price so you don't have to mess with bidding.
Now it would be consider heresy to suggest such a low end bike here in this forum (am I have all higher end ones myself).
However I accidently ordered one of the aluminum ones (mistakenly left it in my cart and well it happened). When it arrived in 3 days by UPS to my suprise, I could not resist giving it a try. After a tune-up check I rode it for about 10 miles and all around the subdivision. I was actually impressed with it. The frame was quite good, the grearing (3x7) was indexed and shifted well. I did feel that the wheels could use a visit to the bike shop for a proper tensioning (but they weren't as bad as many walmart bikes). The fit and finish was actually excellent. No question it is a much better bike all around than any of the many of the Schwins I have owned/ridden and almost as good as a Motobecane classic ones (I own two now and have had at least 4-5 other Moto tandems over the years). Generally the Moto's have better wheels (if you find a good Moto for the price, it would be a good choice ... but they trend toward $600 used).
However the 2bikes on ebay I am referring to above are not a Santana or a Cannondale or any the other top brands. Even used the top brands are way out of there price wise... so ride the $300 one for a few years and save for later if your needs and continued interest dictate a new top brand bike at that time. If it isn't for you they will fetch their original price on Craigs list pretty easy to recoup your investment.
Since your total riding weight will be rather low you are unlikely to expose the ultimate strength differences, etc., that define the top brand bikes, so recommend you give one of these a shot.
Post or PM if you have furhter questions.
/K