In Two-Headed Giant, triggers only once per upkeep, not once for each player.
2005-10-01
For example, a player controls three creatures when Concerted Effort’s ability resolves: one creature with flying and protection from red, one with islandwalk and protection from green; and one with vigilance. All three creatures will have flying, islandwalk, vigilance, protection from red, and protection from green until the end of the turn.
2005-10-01
Creatures keep all abilities granted this way until the end of the turn, even if Concerted Effort or the original creature with that ability leaves the battlefield.
At the beginning of each upkeep, creatures you control gain flying until end of turn if a creature you control has flying. The same is true for fear, first strike, double strike, landwalk, protection, trample, and vigilance.
