This paper presents a quasi-steady model of vibrations of two cylinders in a side-by-side arrangement. The cylinders have flexible support and equal diameters. The model assumes that both cylinders participate in the process of vibration, each of them having two degrees of freedom. The movement of cylinders is described by a set of four non-linear differential equations. These equations are evaluated on the basis of a numerical simulation and experimental data. Moreover many features of cylinder vibrations are found from numerical results and are described in this paper.