RadioHelicopter.com

In conventional helicopters there is usually one main rotor consisting of two or more blades.

However to improve stability some electric radio controlled helicopters are configured for a coaxial setup or twin rotors. The benefit of having two rotors mounted is so they can spin in opposite directions and counter act the torque produced by the spinning rotor. It is also important to note that in a coaxial configuration there is no tail rotor. So you might ask yourself how does a coaxial helicopter change directions with no tail rotor. Well as long as the two rotors are spinning at the same speed the helicopter will hover. As soon as one increases speed torque will be generated again and you will be able to change your direction.

In most coaxial helis the top blades are mounted on the main shaft and the lower blades are mounted on a hollow shaft that runs inside of the main shaft. Twin servos (side by side electric motors) then control one of the two shafts independently.

This design is also used in life sized helicopters as can be seen here of a Russian Air Force Kamov Ka-50 attack helicopter.

Example of an electric coaxial radio controlled  helicopter:

Blade CX2