In the games, Legendary duos are often found after the player enters the Hall of Fame but they usually are not as important to the main storyline as the trios are. Duos or their component parts are featured in several of the movies as keepers of peace.
Sometimes, pairs of Legendary Pokémon once only presented as duos are revealed to actually be Legendary trios in a subsequent game or games of the same generation. However, even within trios, the dynamic between two of the members is often very close to that of a duo: Kyogre and Groudon, Dialga and Palkia, Reshiram and Zekrom, Xerneas and Yveltal, Solgaleo and Lunala are all examples of this.
These duos are pairs of Pokémon with similar stats and movesets, as well as a relationship in lore; the members are usually presented as equals, and function in a symmetrical dynamic.
These duos are made up of Pokémon with a strong relation in the lore, but their stats and movesets tend to be fairly distinct and not equivalent; these duos can be said to be asymmetrical compared to the true duos.
Mewtwo and Mew make up Kanto’s Legendary duo. They are both very elusive, if not unique, Psychic-type Pokémon. Mewtwo was cloned from Mew, meaning the two share DNA and many traits. In the games, however, they are usually treated as unrelated Pokémon, with Mewtwo's stats, level up moves and treatment in the Pokémon Stadium series being closer to that of the tower duo, while Mew's are closer to that of Celebi.
In Mewtwo Strikes Back, Mew embodied innocence and joy, but Mewtwo's experiences as it gained consciousness gave it a jaded and cynical view of humanity. The duality expressed between the two characters formed the basis of their conflict, but innocent Mew, with the help of Ash Ketchum, was able to sway Mewtwo's contempt.
Manaphy and Phione are so far the only two Mythical Pokémon capable of breeding. Each can only breed with Ditto, and Eggs from both Pokémon will produce only Phione. The sea guardian duo has many similarities to the Mew duo; instead of an unnatural (through genetic engineering in a lab), stronger version, Phione is a natural (through breeding), weaker version of Manaphy; both also appear before the "original" in the Pokédex.
Due to conflicting evidence from official sources, Phione's status as a Mythical Pokémon is uncertain; if it is not a Mythical Pokémon, the sea guardian duo is not a Legendary duo.