For sensorless brushless motors the esc times the comutation of the motor by measuring the back emf of each coil winding. Back emf is the voltage generated by the magnets the in motor rotating passed the coils like a generator. This voltage works against the imput voltage from the esc.
EG:
12v put into motor by esc
minus;
3v back emf generated in reverse direction by motor
equals;
9v actually then going into motor.
If we were to take an example of a 3 pole brushless motor, to make the make the motor turn only 2 of the 3 poles are given current at any one time. The esc changes which 2 of the 3 poles is powered up as the motor revolves through 360 degrees. When the motor is turning all the poles are creating an equal amount of back emf voltage towards the esc. The esc reads all the back emf voltages from all the poles. When the esc applies input voltage to 2 of the 3 poles to make the motor turn, the back emf voltage readings of those poles is lower than the back emf voltage of the single pole that isnt being inputted. Because one pole has a different reading to the others the esc from this feedback can tell where the rotor position is in the 360 degree revolution.
By using 2 motors on one esc, It would only work if both motors and back emf readings were perfectly insinc all the time. If just 1 of the 2 motors was slowed down by load, then the back emf readings from the motors would get all crossed and say anything. The esc then would get a jumble of readings and lose the position of the motors, get all the sink wrong and throw a paddy.
Something like that