In a Two-Headed Giant game, Vizkopa Guildmage’s delayed triggered ability causes the opposing team to lose twice the life you gained.

Multiple instances of lifelink on the same creature are redundant.

Each time Vizkopa Guildmage’s second ability resolves, a delayed triggered ability is created. Whenever you gain life that turn, each of those abilities will trigger. For example, if you activate the second ability twice, let those abilities resolve, then gain 2 life, each opponent will lose a total of 4 life.