If two players have each cast Channel Harm targeting a creature the other controls, and a source one player controls would deal damage to the second player or a permanent that player controls, one Channel Harm will prevent that damage and try to deal damage to the creature controlled by the first player. That player’s Channel Harm will then prevent that damage and try to deal damage to the creature controlled by the second player. This forms a loop that either player can break by choosing to not have Channel Harm try to deal damage. It doesn’t matter which player breaks the loop, as no damage will be dealt in either case.