The Double Battle format introduced in Pokémon Ruby and Sapphire lets two Pokémon fight on each side.
In a Double Battle, each Trainer can have two Pokémon active at once. A Trainer may choose moves for both Pokémon during the same turn, and attacks can target an opposing Pokémon, an ally, or all eligible Pokémon depending on the move. This adds positioning, support moves, and team combinations to the standard turn-based system.
Pokémon Ruby and Sapphire were released for Game Boy Advance and formed the core of the franchise’s third generation. Their Hoenn region, new abilities, weather effects, and two-on-two battles expanded the strategic systems established by earlier games.
Double Battles are not the same as Multi Battles. In a Multi Battle, multiple Trainers may participate, usually with each Trainer controlling one active Pokémon. Triple Battles and Rotation Battles were introduced later in specific generations and use different field arrangements.