Recently, the parallel speed control of two induction motors, which uses only one inverter, has been proposed by the vector control method. However, the conventional control method does not consider both the efficiency of the motor control and condition of different load torque to each induction motor (IM). This paper proposes a new high efficient parallel speed control method, which calculations the optimal voltage and frequency value based on non-linear programming method, even when two induction motors on condition different load torque. The non-linear programming method requires the information on load torque and speed reference. Each load torque is estimated by disturbance observer with current simulator and flux simulator. The IM driven system requests only speed sensor using rotary encoder. Hence, it has no current sensor and no voltage sensor. The experimental results point out that both induction motors keep the near same speed control performance with high efficiency in the steady state and the transient state, on condition of different load torque.