  We study synchronization and consensus in a group of dynamical systems
coupled via multiple directed networks. We show that even though the coupling
in a single network may not be sufficient to synchronize the systems,
combination of multiple networks can contribute to synchronization. We
illustrate how the effectiveness of a collection of networks to synchronize the
coupled systems depends on the graph topology. In particular, we show that if
the graph sum is a directed graph whose reversal contains a spanning directed
tree, then the network synchronizes if the coupling is strong enough. This is
intuitive as there is a root node that influence every other node via a set of
edges where each edge in the set is in one of the networks.
