Which Pokémon game introduced battles in which each side uses two Pokémon at once?

Pokémon Ruby and Sapphire introduced battles in which each side uses two Pokémon at once.

These are called Double Battles, and they became one of the defining additions of the third-generation games. Instead of sending out one Pokémon per side, trainers can have two active Pokémon simultaneously. Moves may target one opponent, an ally, or multiple Pokémon depending on their effects.

Ruby and Sapphire were released for the Game Boy Advance in Japan in 2002. They are set in the Hoenn region and also introduced abilities, natures, Pokémon Contests, Pokéblocks, and secret bases. Double Battles gave abilities and move targeting greater strategic importance.

Multi Battles were added alongside Double Battles. In a Multi Battle, two trainers form a team, with each trainer controlling Pokémon on the same side. This differs from an ordinary Double Battle, where one trainer normally controls both active Pokémon.