A Tag-Team Duel (also known as a 2v2 Duel) is a Duel played between two teams of 2 players each. All standard rules apply, and each team is given a shared Life Points counter of 8000 or 16000. See TCG Official Tag Duel Rules as of August 27th, 2013.
When taking part in a Tag-Team Duel, both teams consist of 2 players. Players share a combined Life Point total and their fields. Neither player can look at or use their team-mates or their opponents' hands or Set/face-down cards. If one member of a team has been defeated, that player's Deck and field are unusable.
Players can play in a style similar to Tag Dueling in various video games, such as the World Championship and Tag Force series. In these games, both players share 5 Monster Card Zones, five Spell & Trap Card Zones, a Graveyard, a Banished Zone and a Field Card Zone. The Duel proceeds like a normal Duel, with the Main Deck, Extra Deck, and hand changing as each teammate becomes the new turn player of the team at the start of every other turn. Any player-specific cards will only affect the turn players of each team at the moment, making it entirely possible for a player to get a card owned by a teammate returned to their hand.
The same rules apply for Tag Turbo Duels in Over the Nexus.
Tag-Team Duels in the anime
The first Tag Team Duel in the anime was Joey Wheeler and Yugi Muto vs. the Paradox Brothers. There have also been occurrences of 2 vs. 1 Duels (e.g Yugi and Kaiba vs. Dartz) and even 3 vs. 1 Duels (Tristan, Serenity and Duke vs. Nezbitt in the Virtual World Arc).
Tag-Team Duels are also found in Yu-Gi-Oh! GX. The first was Jaden Yuki and Syrus Truesdale VS the Paradox Brothers, followed by Frost and Thunder VS Syrus Truesdale and Tyranno Hassleberry, then followed by Sarina and a clone of herself VS Jaden Yuki and Aster Phoenix. A Tag-Team tournament was seen in Season 4.
The rules of the Frost/Thunder VS Syrus/Hassleberry Duel are different than most anime tag Duels, since the four players were all given their own Life Point counter.
These rules have been also used in the Yu-Gi-Oh! 5D's series, when Crow Hogan and Yusei Fudo teamed up to face off against Sector Security. Furthermore, even though it took place through a Turbo Duel, the 2008 World Championship rules weren't changed. In the series it has been revealed that this type of Team Dueling is supported even on the world of Turbo Dueling. As there has been shown in the anime the existence of special Duel Runners for Tag-Team Duels.
A 3-on-1 Tag Duel happened from episodes 62 to 64. Rex Goodwin, who was alone, started with 3 times the standard amount of Life Points, or 12000, while Yusei Fudo, Jack Atlas and Crow Hogan had separate fields, Decks and Life Point counters of 4000. Effects of cards used by Rex that targeted the opponent player targeted only one of the three, by his choice.
According to Yu-Gi-Oh! 3D Bonds Beyond Time during the 3 vs. 1 Duel between Yugi, Yusei and Jaden vs Paradox, tag-team players are able to share cards on the field. An example of this is displayed when Yusei allowed Yugi to use the effect of "Reincarnation of Hope" or when Jaden borrowed Yusei's "Junk Gardna" to fuse with "Elemental Hero Neos" to Summon "Elemental Hero Neos Knight". In addition, Spells or Traps could be used to affect monsters that are under your partner's control.
Another is seen in episode 37, this time being a 2v1 match, all players with separated Life Points, and if one player lost, his/her partner would lose as well. It is seen again in episode 42 where Yuma and Kite Dueled against Trey and Quattro.
In Yu-Gi-Oh! ARC-V, a Tag Duel occurs in episode 107 between Gloria Tyler and Grace Tyler against Allen Kozuki and Sayaka Sasayama. In this Duel both players followed the normal Tag Team Rules where each player shared a set of 4000 LP and the same field and Graveyard.
Tag-Team specific cards
Some anime-only cards in the anime can only be used in a Tag Team Duel: