Double Battle/Manga

In The Electric Tale of Pikachu manga
Similar to the, Double Battles appeared in The Electric Tale of Pikachu before they were formally introduced in the games.

Double Battles were formally introduced into Pokémon Adventures in this chapter, though multiple Trainers or Pokémon teaming up were common before.

In Adding It Up with Plusle & Minun I, challenges a  and a  aboard the Abandoned Ship.

In On the Loose and Hyper With Zangoose and Seviper I, Ruby and a battle wild  and.

In You Can Fight Day or Night With Lunatone & Solrock, Tate and Liza explicitly state Double Battle tactics during their battle with Blaise, but are overwhelmed by their opponent's illusions.

During the challenge set by Ultima, and  fight a Double Battle against each other in the Path of Battling. Here, the Trainers are separated by a fence, and the floor they stand on may slide in either direction during the battle, depending on their performance. However, Red and Blue step out from the other end of the path at exactly the same time, meaning that the battle is tied.

In Ring Around the Roserade II, faces Gardenia as a Double Battle.

In Bombastic Bibarel & Heroic Hippopotas, and  battle  Leonardo and  Rebecca at the Seven Stars Restaurant.

In Dramatic Drapion & Crafty Kricketune I, Diamond and Platinum battle two s at Lake Valor.

In Vexing Vespiquen & Unmanageable Mothim, Diamond and Riley battle two s on Iron Island.