From French. Literally, a "movement" of "two." In ballet, it refers to any pairing or duet, although if you call to mind the image of two ballet dancers, a man and a woman, (the danseur noble and the ballerina) you may picture the man holding, lifting, and/or mirroring the ballerina. This "classic" image is known as the grand pas de deux, and it is even broken down into sections: an entree, adage/adagio, a solo (variation) for each dancer, and a coda.
In an equestrian context, it is a dance for two horses and riders. (See dressage).
The term can also be used outside of a dance context, for any situation in which two people or things are working together, or in parallel-- especially when the speaker means to implies a simile (like two dancers) . E.g.,
In Dick's fiction, fakery and truth are in a constant state of flux, trading places and faces, swinging around in a pas de deux. (Bob Goodman. "Minority Retort: Philip K. Dick and the Case of the True Fakes.")
Presidential spokesman Mike McCurry danced a political pas de deux with
reporters, maintaining a studied ignorance of what actually happened between
the president and Monica Lewinsky but faithfully passing on whatever the
president was willing to say.("Tough to Be a Friend of Bill (Editorial), Washington Times 24 August 1998.)
American Ballet Theatre Ballet Dictionary. <http://www.abt.org/education/dictionary/index.html> (4 June 2004)
The American Heritage® Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2000.
Bob Goodman. "Minority Retort: Philip K. Dick and the Case of the True Fakes." 10 July 2002. <http://www.natterbox.com/000019.html> (4 June 2004)