This should be a simple experiment to perform. Resonance involves one frequency, not two. From my experience with interferential electroacupuncture, the primary effects are at F1, F2 and the beat frequency. But since we are using square waves, they contain many other frequencies. . You may find that using a spectrum analyzer to find possible other frequencies.
To start the experiment, you create scar tissue on rats abdomen like has been done before. Then on one rat you show FSM does work to dissolve scar tissue. Then place a bandpass filter on the output. On one rat set the bandpass filter to let only F1 through. See if that works. Then try the same setup with F2. Then with the beat frequency (F2-F1). If none of them work like the base treatment, use the bandpass filter to search for the frequency that is resonating with the scar tissue. This simple experiment should tell us a lot about the interferential aspect of FSM.