Who decides if there are more leaders?

It depends on the reason of the multiple leaders presence. An organization can have multiple leaders when it needs to diversify its aspects to different departments with a leader controlling it. All leader has their own area of authority. But there's always be an executive leader who make the crucial decisions. More than one executive leaders will lead to conflict of interest and confuse the members about who to follow. When an organization has two leaders, the decision of a crucial matter will usually through a grand meeting, involving every important members. So that the decision will be made from common agreement and not biased.