For prop switching you should first be sure that they are rotating in the same direction (not counter-rotating), otherwise you will go backwards instead of forward.
Typically the vibration after a certain rpm only is due to resonance and/or different pitch
of the 2 blades, for example if one blade is slightly warped you may not see with naked eye but the pitch
of one blade would be different to the 2nd blade. Then the rotation at a certain speed will cause vibration (different to vibration due to unbalancing that happens at any speed). Please check also that the stopping point of each blade is not damaged, deformed or missing of stopping rubber inserts