Well I always assumed that Madara simply needed the moon to initially catch multiple people in his infinite Tsukuyomi. I can't remember it being explicitly stated that people had to consistently maintain eye contact (or the like) with the moon in order for them to remain trapped inside the illusion; that's to say, once they've been caught by the moon the first time, they won't need to be caught again - the genjutsu will be 'infinite'.