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This article studies linear-quadratic Stackelberg games between two dominating players (or equivalently, leaders) and a large group of followers, each of whom interacts under a mean field game (MFG) framework. Unlike the conventional major-minor player game, the mean field term herein is endogenously affected by the two leaders simultaneously. These homogeneous followers are non-cooperative, whereas the two leaders can either compete or cooperate with each other, which are respectively formulated as a Nash and a Pareto game. The complete solutions of the leader-follower game can be expressed in terms of the solutions of some non-symmetric Riccati equations. Notably, our analysis suggests that both modes of interactions between leaders has their own merits and neither of them is always more favourable to the community of followers. In our knowledge, a comparative study of the effect of different modes of governance on the society is relatively rare in the existing literature, we here provide its first preliminary quantitative analysis; under a broad class of practically relevant models, we provide sufficient conditions to decide whether cooperation or competition between leaders is more favourable to the followers. Being in common with modern folklore, the relative merits of the two Stackelberg games depend on whether the interests between the two leaders and the followers align among themselves. Representative numerical examples are also supplemented.