Sentence:  Rosencrantz and Guildenstern are a <WSD>double act</WSD> engaged to spy on Hamlet, with the result that he has "at each ear a hearer." 
Meaning of the word marked with <WSD> tags:  A comic pairing in which humor is derived from the uneven relationship between two partners, usually of the same gender, age, ethnic origin and profession, but often with drastically different personalities or behavior, maybe with a stooge or "straight man".