I remember that there have been aircraft desighns that used a long driving shaft for the propeller. Few of these even had counterrotating props. Some of the types ran into kind of severe shaft failures - as far as I remember - caused by kind of harmonics or resonant frequencies ?? I may be corrected at this,
All kinds of vibratory modes/harmonics,coincidence, backward whirl, etc. have been with us for a very long time in gas turbines and Props. Today's engineers have computer models that keep track of the forcing functions and the receptors. Design rules exist to maintain margins between RPM-range drivers and receptors both within and outside the allowed operation limits. In one sense the margins consist of limits on measured vibratory stress while in the case of higher speeds than normal a healthy margin in speed between normal operation and a dangerous vibration (GE CF6, National Airlines)
The problem comes in when operation is either abnormal (damaged parts creating unbalance or aerodynamic burbles) or parts which have lower than expected fatigue margin due to manufacturing flaws. And one of the largest corntributors have been misassembled parts which seriously affect the stiffness of joints between parts.
Nothing that RR doesn't already know and adresses