Two-Headed Giant (also known as 2HG) is a DCI-sanctioned multiplayer format for Magic: the Gathering in which 2 Teams of 2 players each all play together in a single game.   
Each team sits together on one side of the table. Each team decides the order in which its players sit. The player seated on the right within each team is the primary player, and the player seated on the left is the secondary player. Each team has a shared life total, which starts at 40 life.
Two-Headed Giant Constructed decks must contain a minimum of sixty cards. There is no maximum deck size; however, you must be able to shuffle your deck with no assistance. Teammates may review each other's hands and discuss strategies at any time. Each team takes turns rather than each player. Both the players on a team take their team turn at the exact same time.
Two-Headed Giant uses the Unified Deck Construction rules (with the exception of basic land cards, a team's combined decks may not contain more than four of any individual card). Sideboards are not allowed in Constructed Multiplayer tournaments. If a card is [[restricted in a particular format, no more than one of that card may be used by the team. No players may use cards that are banned in a particular format.
In addition to the above rules, the following card is banned in Constructed Two-Headed Giant tournaments:
From the Comprehensive Rules (Khans of Tarkir)
- 810. Two-Headed Giant Variant
- 810.1. Two-Headed Giant games are played with two teams of two players each.
- 810.2. The Two-Headed Giant variant uses the shared team turns option. (See rule 805.)
- 810.3. Each team sits together on one side of the table. Each team decides the order in which its players sit.
- 810.4. Each team has a shared life total, which starts at 30 life.
- 810.5. With the exception of life total and poison counters, a team’s resources (cards in hand, mana, and so on) are not shared in the Two-Headed Giant variant. Teammates may review each other’s hands and discuss strategies at any time. Teammates can’t manipulate each other’s cards or permanents.
- 810.6. The team who plays first skips the draw step of its first turn.
- 810.7. The Two-Headed Giant variant uses different combat rules than other multiplayer variants.
- 810.7a Each team’s creatures attack the other team as a group. During the combat phase, the active team is the attacking team and each player on the active team is an attacking player. Likewise, the nonactive team is the defending team and each player on the nonactive team is a defending player.
- 810.7b Any one-shot effect that refers to the “defending player” refers to one specific defending player, not to both of the defending players. The controller of the effect chooses which one the spell or ability refers to at the time the effect is applied. The same is true for any one-shot effect that refers to the “attacking player.”
Any characteristic-defining ability that refers to the “defending player” refers to one specific defending player, not to both of the defending players. The controller of the object with the characteristic-defining ability chooses which one the ability refers to at the time the nonactive players become defending players.
All other cases in which the “defending player” is referred to actually refer to both defending players. If the reference involves a positive comparison (such as asking whether the defending player controls an Island) or a relative comparison (such as asking whether you control more creatures than the defending player), it gets only one answer. This answer is “yes” if either defending player in the comparison would return a “yes” answer if compared individually. If the reference involves a negative comparison (such as asking whether the defending player controls no black permanents), it also gets only one answer. This answer is “yes” if performing the analogous positive comparison would return a “no” answer. The same is true for all other cases that refer to the “attacking player.”
- 810.7c As the declare attackers step begins, the active team declares attackers. If an effect of an object controlled by a defending player prohibits a creature from attacking him or her, that creature can’t attack the defending team. The active team has one combined attack, and that set of attacking creatures must be legal as a whole. See rule 508.1.
- 810.7d As the declare blockers step begins, the defending team declares blockers. Creatures controlled by the defending players can block any attacking creatures. The defending team has one combined block, and that set of blocking creatures must be legal as a whole. See rule 509.1.
- 810.7e Once blockers have been declared, for each attacking creature that’s become blocked by multiple creatures, the active team announces the damage assignment order among the blocking creatures. Then, for each creature that’s blocking multiple creatures, the defending team announces the damage assignment order among the attacking creatures.
- 810.7f As the combat damage step begins, the active team announces how each attacking creature will assign its combat damage. If an attacking creature would assign combat damage to the defending team, the active team chooses only one of the defending players for that creature to assign its combat damage to. Then the defending team announces how each blocking creature will assign its combat damage. See rule 510.1.
- 810.8. The Two-Headed Giant variant uses the normal rules for winning or losing the game (see rule 104), with the following additions and specifications.
- 810.8a Players win and lose the game only as a team, not as individuals. If either player on a team loses the game, the team loses the game. If either player on a team wins the game, the entire team wins the game. If an effect would prevent a player from winning the game, that player’s team can’t win the game. If an effect would prevent a player from losing the game, that player’s team can’t lose the game.
- 810.8b If a player concedes, his or her team leaves the game immediately. That team loses the game.
- 810.8c If a team’s life total is 0 or less, the team loses the game. (This is a state-based action. See rule 704.)
- 810.8d If a team has fifteen or more poison counters, that team loses the game. (This is a state-based action. See rule 704.)
- 810.9. Damage, loss of life, and gaining life happen to each player individually. The result is applied to the team’s shared life total.
- 810.9a If a cost or effect needs to know the value of an individual player’s life total, that cost or effect uses the team’s life total instead.
- 810.9b If a cost or effect allows both members of a team to pay life simultaneously, the total amount of life they pay may not exceed their team’s life total. (Players can always pay 0 life.)
- 810.9c If an effect sets a single player’s life total to a specific number, the player gains or loses the necessary amount of life to end up with the new total. The team’s life total is adjusted by the amount of life that player gained or lost.
- 810.9d If an effect would set the life total of each player on a team to a number, that team chooses one of its members. On that team, only that player is affected.
Example: In a Two-Headed Giant game, one team has 7 life and the other team has 13 life. A player casts Repay in Kind, which reads, "Each player's life total becomes the lowest life total among all players." Each team chooses one of its members to be affected. The result is that the chosen player on the team that has 13 life loses 6 life, so that team's life total winds up at 7.
- 810.9e A player can’t exchange life totals with his or her teammate. If an effect would cause that to occur, the exchange won’t happen.
- 810.9f If an effect instructs a player to redistribute any number of players’ life totals, that player may not affect more than one member of each team this way.
- 810.9g If an effect says that a player can’t gain life, no player on that player’s team can gain life.
- 810.9h If an effect says that a player can’t lose life, no player on that player’s team can lose life or pay any amount of life other than 0.
- 810.10. Effects that cause players to get poison counters happen to each player individually. The poison counters are shared by the team.
- 810.10a If an effect needs to know how many poison counters an individual player has, that effect uses the number of poison counters that player’s team has. If an effect needs to know how many poison counters a player's opponents have, that effect uses the number of poison counters opposing teams have.
- 810.10b If an effect says that a player loses poison counters, that player’s team loses that many poison counters.
- 810.10c If an effect says that a player can’t get poison counters, no player on that player’s team can get poison counters.
- 810.10d A player is “poisoned” if his or her team has one or more poison counters.
- 810.11. The Two-Headed Giant variant can also be played with equally sized teams of more than two players. For each player a team has beyond the second, that team’s starting life total is increased by 15 and the number of poison counters required for the team to lose is increased by five. (These variants are called Three-Headed Giant, Four-Headed Giant, and so on.)
- ↑ Laura Mills and Anthony Alongi. (August 15, 2005.) "The Two-Headed Article", Daily MTG, magicthegathering.com, Wizards of the Coast.
- ↑ Wizards of the Coast. (not dated.) "Casual Formats", Daily MTG, magicthegathering.com, Wizards of the Coast.
- ↑ Kelly Digges. (March 30, 2009.) "By the Numbers", Daily MTG, magicthegathering.com, Wizards of the Coast.