TANDEM

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.

(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.

A group of two or more people, machines etc. working together; close collaboration.

Together; working as one.

"Their skillful tandem work made the project quick and successful."

One behind the other.

"The horses were harnessed tandem."