A theoretical model has been developed for studying the response of the phase Doppler interferometer when multiple particles are simultaneously present within the measurement probe volume. The developed model incorporates the geometrical optics theory for describing the coherent interaction between the scattered light signals of multiple particles, each having different size, velocity, trajectory, and arrival time. The resulting Doppler signal is processed by a theoretical signal processor which can simulate the performance characteristics of different signal processing schemes that are widely used in phase Doppler interferometry, namely, zero-crossing counter, covariance, autocorrelation, and DFT processors. The application of the developed model for studying the coherent scattering by two particles has been specifically addressed in this paper. It has been shown that a DFT processor can be used to simultaneously measure the size and velocity of the two particles in most instances. However, for more than two particles, the signal processing scheme becomes more complex because of a quadratic increase in the beat frequency components.