Phase difference of two signal of different frequency Currently, I have two signals, the main components of both signals are 60Hz, but both also have weaker response at 180Hz + small amount of noise. As shown in the photo below, I want to find the phase difference between the 60Hz component of first signal, and the 180Hz component of the second signal. 

As you can see the second waveform of the figure, the phase lagg is of the 180Hz signal is 1.39ms, which coresponds to a 90 degree phase shift. 



I am wondering, if I want to implment this phase measurement, could I simply take the fft of two signals, and then find the phase at 60Hz of signal 1, and subtract the phase at 180Hz of signal 2?

Or if there is better way to do so?
if im not wrong this figure is from "UNIQUE ASPECTS OF DISTRIBUTION SYSTEM HARMONICS DUE TO HIGH IMPEDANCE GROUND FAULTS"

Actually, measuring the phase difference between two different frequency signals does not make sense. imagining two rotating sticks, one is faster than another one, the phase difference keeps changing.
if im not wrong this figure is from "UNIQUE ASPECTS OF DISTRIBUTION SYSTEM HARMONICS DUE TO HIGH IMPEDANCE GROUND FAULTS"

Actually, measuring the phase difference between two different frequency signals does not make sense. imagining two rotating sticks, one is faster than another one, the phase difference keeps changing.