Which Pokémon game first introduced battles with two Pokémon on each side?

Pokémon Ruby and Sapphire first introduced battles with two Pokémon on each side.

Released for the Game Boy Advance, Ruby and Sapphire added Double Battles as a major change to the established battle system. In these battles, each side can have two Pokémon active at the same time, allowing moves and abilities to affect allies, opponents, or multiple targets.

The games were first released in Japan in late 2002 and internationally in 2003. They also introduced Pokémon Abilities, Pokémon Natures, Pokémon Contests, and 135 new species in the Hoenn region.

Pokémon Emerald, released later in the same generation, also features Double Battles and expands their use in several important encounters. However, Emerald was not the origin of the mechanic. Ruby and Sapphire are the pair credited with introducing it to the main series.