A solution is proposed to the upcrossing problem for a stress of a special form encountered in probabilistic torsional vibration analysis of marine Diesel engine shafting systems. For the case that two engine criticals occur at the same engine speed, the stress is modeled as a cyclostationary random process with two frequencies. An equivalent stationary process is defined, requiring that its envelope process has the same upcrossing rate as the envelope of the original cyclostationary random process. It is assumed that both processes have the same probability of upcrossing a specific threshold in a given time interval. Numerical implementation shows the applicability and efficiency of the method.