Take away the gizmos and it is not so easy. One way of preventing the rear rotor from catching up with the front is to differentiate the collective and cyclics. Basically if the rear rotor isn't leaning as far forward as the front it wont catch up. This means it hasn't the same vertical component so it needs more pitch. I believe that on the early Piaseckis or Vertols there was a spirit level in the cockpit to assist you to level it with differential collective.
The problem with twin rotors is the transmission. Apart from the Belvedere one or more of the gearboxs was dependent on a shaft to drive it. This breaks and you have a fatal disymmetry of lift. Luckily AFIK this hasn't happened yet.
The Belvedere had an engine under each rotor and a synchronisation shaft keeping the blades apart. This has broken twice in service. In both cases the pilots flew the aircraft so the rear rotor, turning at a different speed than the front, was stepped above the front until they landed.
The shutdown was quite noisy.