We present a detailed theoretical study of a vertical-cavity surface-emitting laser (VCSEL) with two optically coupled, active cavities. The study is based on a rate-equation model written for carriers and photons under steady-state conditions. The model allows one to determine all the relevant parameters-carrier densities, gains, and output powers-starting from two input parameters: the injection currents in each cavity. The system of equations is solved for different operating regimes of the device and the results provided by the model are shown to be in very good qualitative and quantitative agreement with the experimental data.