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Some games use the idea of a duo to try and enhance the gameplay totally uneccessarily - i.e platformers where one character has different moves to another so you need to swap character to complete a certain level. The game would be no different with one character who had a bigger range of moves.
There are plenty of great games out there without duo's involved to indicate that duo's in themselves are not an important factor, but used wisely they can add to the experience.