When applying phase-Doppler anemometry (PDA) to measurements of mass flux, one has to deal with errors caused by multiple scattering effects within a PDA measurement volume. Theoretical research on the influence of multiple scattering effects on PDA is more promising than experimental work, because each error source can be treated separately. Therefore, we have simulated multiple scattering effects occurring in the measurement volume by extending a method for light scattering calculations to Gaussian beams: the multiple multipole method (MMP). The current version of our computer program is able to simulate PDA with any set-up consisting of Gaussian beams, using one or more homogeneous particles with arbitrary shape and movement and two detectors at any position and with an arbitrarily shaped aperture. In this paper we have concentrated on multiple scattering effects caused by two equally sized spheres, a case which is frequently found in atomization.