In a Two-Headed Giant game, Celestial Mantle’s essentially doubles the team’s life total. Specifically, the triggered ability will affect one player’s life total, and then the team’s life total is adjusted by the amount of life the player gains as a result of this ability. Suppose a creature enchanted by Celestial Mantle deals combat damage to an opponent, and that creature’s controller’s team has 11 life. That player then has 11 life, so it’s doubled to 22, for a net gain of 11 life. The team’s life total becomes 22 (11 + 11).