Enchantment — Aura
Enchant creature Enchanted creature gets +3/+3. Whenever enchanted creature deals combat damage to a player, double its controller's life total.
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).
If the enchanted creature’s controller has a life total below 0 (which is possible if that player controls Platinum Angel, for example), Celestial Mantle’s triggered ability multiplies that number by two. For example, if the player had -4 life, that player’s life total becomes -8.
At the time the ability triggers, determine which creature Celestial Mantle is enchanting. As the ability resolves, determine who currently controls that creature (or, if it’s no longer on the battlefield, determine who controlled it when it left). That’s the player whose life total is doubled. It doesn’t matter who controls Celestial Mantle, who controlled the creature at the time the ability triggered, or what creature Celestial Mantle enchants at the time the ability resolves.