A method of protecting a multi-rotor wind turbine support structure against vibration induced overload, wherein at least two rotor arms are attached to the wind turbine support structure, and the wind turbine comprising at least a first nacelle with a first rotor and a second nacelle with a second rotor, the first nacelle and the second nacelle being carried by the respective rotor arms at substantially equal distances from a central longitudinal axis of the wind turbine support structure, the method comprising the steps of: receiving motion data ( ÿ 0, ÿ 1, ÿ 2) representing a movement of the first nacelle and of the second nacelle the motion data ( ÿ 1, ÿ 2) is obtained by at least two motion sensors (108, 109, 114, 124), located along the respective rotor arms, characterized in that the method further comprises: combining the motion data relating to the first and the second nacelle in order to decouple a torsion signal from a fore-aft bending signal; determining a vibrational mode level of a torsion mode by subtracting the motion signal ( ÿ 1) from the motion sensor at the first nacelle from the motion signal ( ÿ 2) from the motion sensor at the second nacelle, determining a vibrational mode level of a fore-aft bending mode by summing the motion signal ( ÿ 1) from the motion sensor at the first nacelle and the motion signal ( ÿ 2) from the motion sensor at the second nacelle, and activating a safety mode if at least one of the vibrational mode levels exceeds a respective vibrational mode threshold. receiving motion data ( ÿ 0, ÿ 1, ÿ 2) representing a movement of the first nacelle and of the second nacelle the motion data ( ÿ 1, ÿ 2) is obtained by at least two motion sensors (108, 109, 114, 124), located along the respective rotor arms, characterized in that the method further comprises: combining the motion data relating to the first and the second nacelle in order to decouple a torsion signal from a fore-aft bending signal; determining a vibrational mode level of a torsion mode by subtracting the motion signal ( ÿ 1) from the motion sensor at the first nacelle from the motion signal ( ÿ 2) from the motion sensor at the second nacelle, determining a vibrational mode level of a fore-aft bending mode by summing the motion signal ( ÿ 1) from the motion sensor at the first nacelle and the motion signal ( ÿ 2) from the motion sensor at the second nacelle, and activating a safety mode if at least one of the vibrational mode levels exceeds a respective vibrational mode threshold. combining the motion data relating to the first and the second nacelle in order to decouple a torsion signal from a fore-aft bending signal; determining a vibrational mode level of a torsion mode by subtracting the motion signal ( ÿ 1) from the motion sensor at the first nacelle from the motion signal ( ÿ 2) from the motion sensor at the second nacelle, determining a vibrational mode level of a fore-aft bending mode by summing the motion signal ( ÿ 1) from the motion sensor at the first nacelle and the motion signal ( ÿ 2) from the motion sensor at the second nacelle, and activating a safety mode if at least one of the vibrational mode levels exceeds a respective vibrational mode threshold.