Single rotor and twin rotor helicopters work on the same principle: The engines drive a gearbox which then drives two rotor systems. On the so-called single rotor system a big one holds it up and a small vertical one stops the fuselage spinning with the torque and on the twin rotor two middle sized rotors hold it up and each stops the fuselage spinning because they go in opposite directions.
Modern helicopters have a long shaft that connects one of the rotors to the gearbox. Should that break then you are in big trouble single or twin. Singles have been known to get away with it but with twin the sudden dissymmetry between the two rotors is normally fatal.
Fareastdriver: Five years on fast jets and then forty three years on rotary. The best move I ever made.