The classic problem of the existence of genuine interface plasmons at the junction between two free-electron metals is revisited. By applying Bloch's standard hydrodynamical model for the homogeneous free-electron gas to the case of a bi-metal system, we demonstrate that localized interface plasmons resonantly coupled to surface plasmons do exist. It is shown that the amplitude of the resonant field at the metal-metal interface strongly depends on the layer thickness and disappears for a semi-infinite system.