You have three options when it comes to adding a folding tandem bicycle to your bike collection. Your first option is to buy a folding tandem bicycle. The manufacturers that make folding tandem bikes include: Bilenky, Calfee, Davidson and Co-Motion. Some of the models you may be interested in include: the Bilenky Cosmo-Comfort Travel Tandem, the Bilenky Siganture Tinker, ariZona Tandem Davis Double, the Bob Brown, the Burly rumba S & S Tandem, the Burly Duet S & S and the Bushnell Road Tandem.
Your second option is to get your regular tandem retrofitted with a special coupling device called the S & S Coupling(TM). This device is a lug nut that allows for quick connecting and disconnecting of tandem bike sections. If this is an option that you are interested in, then you will want to take your tandem to a bike mechanic or bike shop that has experience with this bike modification.
The third option that you have is to buy a trail-a-bike system. These half bike trailers are great for families. They can turn just about any adult bike into a folding tandem. The trailer can be attached or detached as needed.
One of trail-a-bike products that is available is the Chariot Catch'em Trail-a-Bike. This tandem add on is designed for children between the ages of 5 and 8. It connects to an adult bike with a secure coupling. It has its own 6 speed shifting set-up which will help children learn how to shift their bike when their ready to go out on their own. This tandem trailer retails for under 0.