Rule 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. Example: In a Two-Headed Giant game, a player on a team that has 17 life is targeted by Beacon of Immortality, which reads, in part, “Double target player’s life total.” That player gains 17 life, so the team winds up at 34 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 if their team’s life total is 50 or more. Example: In a Two-Headed Giant game, a player on a team that has 11 life controls Lurking Evil, an enchantment that reads, “Pay half your life, rounded up: Lurking Evil becomes a 4/4 Horror creature with flying.” To activate the ability, that player must pay 6 life. The team winds up at 5 life.