Tandem describes an arrangement where people or animals are in formation, one behind the other. Couples attached at the hip sometimes like to ride tandem bicycles, which allow two people to pedal the same bike. How cute.
In 1785, tandem first came into existence as a noun meaning “carriage pulled by horses harnessed one behind the other.” About a hundred years later, people started to use tandem to refer to bicycles with two seats. This type of bicycle has two sets of pedals and two seats, but only one set of wheels. To transform tandem into an adverb, just stick in in front of it. "In tandem" can just mean "together" — like two departments working in tandem on a project.