Pokémon Ruby and Sapphire introduced Double Battles, in which two Pokémon battle on each side at the same time.
The third-generation games added this format to the main series when they launched for Game Boy Advance in Japan in 2002. Instead of each trainer sending out one Pokémon, a Double Battle places two active Pokémon on each side, creating interactions between moves, abilities, targets, and positioning.
Double Battles became a lasting part of Pokémon strategy. Moves such as Protect and Earthquake could affect the tactical choices of both allies and opponents, while abilities and combinations gained importance because two Pokémon acted together. The format also became central to official competitive play, including the Pokémon Video Game Championships.
Triple Battles and Rotation Battles arrived later in Pokémon Black and White. Raid Battles are a different cooperative format introduced in Pokémon Sword and Shield, so they should not be confused with the feature first established by Ruby and Sapphire.