Independent Coasting System
http://www.davincitandems.com/dv2.html
Starting, stopping and going through corners are as easy as on a single bike.
Captain and stoker can independently relax, take a drink, adjust seat position, stand for a few seconds to relieve weary sit bones, even take a moment to gaze at scenery, while the other keeps pedaling.
Traversing obstacles is easier and more efficient because the captain and stoker can coordinate avoiding the obstacles while maintaining forward momentum.
Asynchronous pedaling decreases strain on the final drive train.