Separation of overlapping frequencies
In case your signal is fully stationary, just go ahead and use a "sinusoidal fitting" and take out the frequency that is not off your interest.
Then you have a disturbance free signal. Although since these signals are, I guess, not multiple integers of each other, your main frequency will also be low pass filtered. You can also use a notch filter, which will have similar behavior.

Please update us if you found a good answer.