Advanced diffusion-weighted MRI allows non-invasive cell size and volume mapping in cancerous tumours. Existing methods assume that a given voxel contains one cell population. This work investigates the feasibility of estimating the radii and volume fractions of 2 cell populations co-existing in the same voxel and introduces techniques to improve the robustness and stability of fitting. The influence of noise on the fitted parameters is also examined for a range of SNR values. Reliable estimation of cell size and volume for tissue with 2 cell populations opens exciting avenue of potential applications in both tumor diagnosis and treatment monitoring.