  We report coherent coupling between two macroscopically separated
nitrogen-vacancy electron spin ensembles in a cavity quantum electrodynamics
system. The coherent interaction between the distant ensembles is directly
detected in the cavity transmission spectrum by observing bright and dark
collective multiensemble states and an increase of the coupling strength to the
cavity mode. Additionally, in the dispersive limit we show transverse
ensemble-ensemble coupling via virtual photons.
