Rule 806.10a

If an effect needs to know the value of an individual player's life total, that effect uses the team's life total divided by two, rounded up, instead. Example: In a Two-Headed Giant game, a team is at 17 life when a player activates Heartless Hidetsugu's ability, which reads, "Heartless Hidetsugu deals to each player damage equal to half that player's life total, rounded down." For the purposes of this ability, each player on that team is considered to be at 9 life. Heartless Hidetsugu deals 4 damage to each of those players, for a total of 8 damage. The team will end up at 9 life. Example: In a Two-Headed Giant game, a player controls Test of Endurance, an enchantment that reads, "At the beginning of your upkeep, if you have 50 or more life, you win the game." At the beginning of that player's upkeep, the player's team wins the game only if his or her share of the team's life total is 50 or more. The team's life total must be 99 or more for that to happen. Example: In a Two-Headed Giant game, a player controls Lurking Jackals, which reads, "When an opponent has 10 life or less, if Lurking Jackals is an enchantment, it becomes a 3/2 Hound creature." If the opposing team has 22 life and 1 damage is dealt to a particular opponent, Lurking Jackals won't become a creature. The opposing team's life total must be 20 or less for that to happen.

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