an arrangement of two or more objects or persons one behind another
"The cyclists rode in tandem, positioning themselves one directly behind the other to conserve energy on the long climb."
a bicycle with two sets of pedals and two seats
"The children rode their tandem down the hill, taking turns to pedal while the other steered."
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.
"The heavy farm cart was drawn across the muddy field by a tandem of draft horses working together to pull the load."
In plain English: A tandem is two things connected together so they work as one unit, like bicycles built for two people to ride side by side or back to back.
"The two cyclists rode in tandem to save energy while climbing the hill."
Usage: Use "tandem" to describe people or machines operating in a coordinated line rather than side by side. Avoid using it for items that are merely adjacent without synchronized function.
Together; working as one.
"The two cyclists rode in tandem, matching their speed and rhythm perfectly."
One behind the other.
"The two cyclists rode in tandem, with the second rider directly behind the first to reduce wind resistance."
The word tandem comes from Latin, where it originally meant "at last" or "finally." English speakers humorously applied this meaning of doing something in a single line to describe two horses harnessed one behind the other instead of side by side.