Effects that would replace gaining life with another effect won’t apply because it’s impossible for players to gain life.

Spells and abilities that would cause a player to gain life still resolve, but the life-gain part has no effect.

If an effect says to set a player’s life total to a certain number and that number is higher than the player’s current life total, that part of the effect won’t do anything. (If the number is lower than the player’s current life total, the effect will work as normal.)

The amount of life lost is calculated when the triggered ability resolves. For example, if a player has 20 life and there are two Havoc Festivals on the battlefield, the first ability will cause that player to lose 10 life and then the second ability will cause that player to lose 5 life.

In a Two-Headed Giant game, the last ability triggers for each player. Any life loss is applied to the team’s life total. For example, if the team has 30 life, the first ability will cause the player (and thus the team) to lose 15 life and then the second ability will cause the other player (and thus the team) to lose 8 life. The team will end up having 7 life.