Pokémon Red Version, Blue Version, and Yellow Version - Tag Team Battles
In Pokémon Stadium (known as Pokémon Stadium 2 in Japan), two players can team up on a single side. Each player on a side enters six Pokémon, then chooses three of them before a battle begins. Each player on a side with two players can choose only a Pokémon he or she controls when it switches out. If all Pokémon on a single side faint, that side loses.
Things such as same-species and same-item restrictions are enforced only for each player’s team individually, not for the combined team when a battle begins.