Tandem adv. & a. : One after another; -- said especially of horses harnessed and driven one before another, instead of abreast.

Tandem n. : A team of horses harnessed one before the other.

Example:
Tandem nursing: How is it the milk is suitable for both the newborn and the older child?

Taxonomy
Tandem, free encyclopedia, tandem seating, Oxford English Dictionary, working together, 01-28, free dictionary, driving, tandem bicycle, Tandem language learning, search, Ponies, side-by-side, extension, animals, adjective, two-seat aircraft, occupants.

Defination from the web:
tandem is often used as an adjective in "tandem seating," as in a two-seat aircraft where the two occupants sit one behind the other, ... Tandem is also used as a term for the wheels on a semi trailer to balance the weight of a load of freight. ...