CNGS muon monitoring: Why do we need both muon detector stations at the start-up of the CNGS neutrino beam ?

The CNGS design has been approved with two muon detector stations, separated by 67 metres of rock. During the recent discussions on cost reduction, it has been suggested that a possible saving would be to postpone equipping the second muon detector station. The importance of the second array of muon detectors, in particular at the start-up of CNGS, is being described in this note. It is argued that both muon detector stations must be operational from the beginning of CNGS operation.