Abstract Two-layer shallow-water flow without mixing is discussed. The existence of two types of waves (surface and internal) with rather different velocities of propagation leads to a numerical approach with two separate grids. For that purpose, the differential equations are written in a weak-interaction form. Particular attention is paid to the formulation of boundary conditions leading to a mathematically well-posed and numerically stable problem. A slightly modified leap-frog method is shown to satisfy the requirements. Some examples are given, including salt-water intrusion in a schematic estuary.