noun

• (countable) A carriage pulled by two or more draught animals (generally draught horses) harnessed one behind the other, both providing pulling power but only the animal in front being able to steer.
  "[H]e was a man whose head was at this time entirely full of gigs, and tandems, and unicorns: business was his aversion; pleasure was his business."
• (by extension, countable)
  "Later in the day the Row is empty, but, on the other hand, the pleasure horse monopolises the circle, now comparatively deserted. Singly, in pairs, tandems, and fours, he draws the family carriage with a lozenge on the panels, the brougham of the fashionable doctor, the coupé of the popular actress, the man about town's smart dog-cart."
• (by extension, countable)
  "If you want a canoe that can seat both of you comfortably, you’ll need a tandem."
• (by extension, countable)

adv

• (figurative) In close collaboration; collaboratively, cooperatively.

adj

• (figurative) Working together collaboratively; collaborative, cooperative.
  "Their skillful tandem work made the project quick and successful."

verb

• (ambitransitive) Synonym of tandemize (“(transitive) to harness or drive (two draught animals, generally draught horses) one behind the other; to set up (two or more things, such as pieces or equipment) to work in tandem or together; (intransitive) to drive a tandem (‘carriage pulled by two or more draught animals (generally draught horses) harnessed one behind the other’)”)
  "[O]ur dog-cart having come to hand, from the hilliness of the road, we tandemed the ten miles due west, along the southern shore of Clew Bay."