We explore the photon population dynamics in two coupled circuit QED systems. For a sufficiently weak
intercavity photon hopping, as the photon-cavity coupling increases, the dynamics undergoes double
transitions first from a delocalized to a localized phase and then from the localized to another delocalized
phase. The latter delocalized phase is distinguished from the former one; instead of oscillating between the
two cavities, the photons rapidly quasiequilibrate over the two cavities. These intriguing features are
attributed to an interplay between two qualitatively distinctive nonlinear behaviors of the circuit QED
systems in the utrastrong coupling regime, whose distinction has been widely overlooked.