A carriage pulled by two or more draught animals (generally draught horses) harnessed one behind the other, both providing the pulling power but only the animal in front able to steer. [from mid 18th c.]
(transferred sense) A bicycle or tricycle in which two people sit one behind the other, both able to pedal but only the person in front able to steer. [from late 19th c.]
(figuratively) A group of two or more people, machines etc. working together; close collaboration.
(education) A method of language learning based on mutual exchange, where ideally each learner is a native speaker in the language the other person wants to learn