Most people realize that the forks on a tandem resist doubled weight and doubled force. But there are actually several hidden factors forcing tandem forks to work even harder.
First, a tandem fork takes more hits. On a single bike, when you see an unavoidable bump, you "lift and sail' to minimize the impact. On a tandem this just isn't possible. Tandem forks endure hundreds of intensified bumps on every ride.
The next factor is braking. While front brakes do over 80% of the work on all vehicles, the braking force of a single bike is limited by weight distribution. If a rider applies the front brake too hard, he goes over the bars. On a tandem, because weight and wheel base practically eliminate the potential for over the bar flights, every tandem can theoretically stop shorter, other factors (such as tire surface in contact with the road per pound) being proportionately equal. However, fast stops will only be realized if the fork can withstand the force of a fully applied front brake - without shuddering.
Finally, we must consider the challenge of faster descents. Because tandems have doubled mass yet roughly only the same frontal area as a single, tandems can descend up to 50% faster. During a fast descent the laws of physics say the force (f) required to stop a descending tandem with doubled mass (2m) and 50% higher velocity (1.5v) is actually 450% as great (f=mv2). To descend with confidence on tandems you need strong forks.