In many applications of noise control, the greatest annoyance is caused by periodic low frequency noise. Successful reduction of such noise can often be achieved by using an Active Noise Control (ANC) system with narrowband feedforward control. If several noise sources contribute to the sound field, a multiple--reference control system is usually required. This allows the reference signals from each noise source to be processed individually within the controller, thereby enabling individual control of the sound field from each noise source. The present paper addresses the problem of controlling noise from two sources that are more or less synchronized. A typical application is the control of propeller-generated noise within a twin propeller aircraft. To find out whether a multiple-reference controller is necessary, or if a single--reference controller can do the job, the performance of a single- versus twin-tacho control algorithm was evaluated in a comparative study. The study is a computer simulation using real life data recorded in a Dornier 328 under different flight conditions. The results demonstrate that the twin-tacho algorithm performs better than the single-tacho whenever there is a slight deviation in the rotational speed of the two propellers.