  We extend the observability model to multiplex networks. We present
mathematical frameworks, valid under the treelike ansatz, able to describe the
emergence of the macroscopic cluster of mutually observable nodes in both
synthetic and real-world multiplex networks. We show that the observability
transition in synthetic multiplex networks is discontinuous. In real-world
multiplex networks instead, edge overlap among layers is responsible for the
disappearance of any sign of abruptness in the emergence of the the macroscopic
cluster of mutually observable nodes.
