A tail rotor isolation system for rotorcraft includes a secondary engine, first and second freewheeling units, an isolation assembly and a tail rotor system. The secondary engine is coupled to the input race of the first freewheeling unit. A main rotor system is coupled to the output race of the second freewheeling unit. The isolation assembly is coupled to the output race of the first freewheeling unit and has a fully engaged position coupling the input and output races of the second freewheeling unit and a partially engaged position coupled to the input race but decoupled from the output race of the second freewheeling unit. The tail rotor system is coupled to the input race of the second freewheeling unit such that in the partially engaged position of the isolation assembly, the overrunning mode of the second freewheeling unit isolates the tail rotor system from the main rotor system.