Complex contagion models have been developed to understand a wide range of social phenomena such as adoption of cultural fads, the diffusion of belief, norms, and innovations in social networks, and the rise of collective action to join a riot. Most existing works focus on contagions where individuals' states are represented by {\em binary} variables, and propagation takes place over a single isolated network. However, characterization of an individual's standing on a given matter as a binary state might be overly simplistic as most of our opinions, feelings, and perceptions vary over more than two states. Also, most real-world contagions take place over multiple networks (e.g., Twitter and Facebook) or involve {\em multiplex} networks where individuals engage in different {\em types} of relationships (e.g., acquaintance, co-worker, family, etc.). To this end, this paper studies {\em multi-stage} complex contagions that take place over multi-layer or multiplex networks. Under a linear threshold based contagion model, we give analytic results for the probability and expected size of \textit{global} cascades, i.e., cases where a randomly chosen node can initiate a propagation that eventually reaches a {\em positive} fraction of the whole population. Analytic results are also confirmed and supported by an extensive numerical study. In particular, we demonstrate how the dynamics of complex contagions is affected by the extra weight exerted by \textit{hyper-active} nodes and by the structural properties of the networks involved. Among other things, we reveal an interesting connection between the assortativity of a network and the impact of \textit{hyper-active} nodes on the cascade size.