Each player's life total becomes the lowest life total among all players.
For a player’s life total to become a certain number that’s lower than their current life total, what actually happens is that the player loses the appropriate amount of life. For example, if the lowest life total in the game is 5 and another player has 12 life, Repay in Kind will cause that player to lose 7 life. Abilities that interact with life loss will interact with this effect accordingly.
In a Two-Headed Giant game, anything that cares about a player’s life total checks the life total of that player’s team. In addition, if an effect would cause the life total of each member of a team to become a certain number, that team chooses one of its members. On that team, only that player is affected, then the life total change is applied to the entire team. For example, if one team has 13 life and another team has 21 life, the team with the higher life total chooses one of its members and Repay in Kind causes that player to lose 8 life, thus causing that team’s life total to also be 13.