We present a scheme for coupling a 2D lattice Boltzmann free-surface solver with a 1D lattice Boltzmann shallow water solver, allowing to save computational effort and efficiently realize multiscale systems. The accuracy of the coupling is validated with two tests. First, we compare the numerical and analytical solutions in a setup with fixed inflow current and outflow water level in a canal. Secondly, the physics of wave propagation and reflection in a domain is investigated in a coupled simulation, and compared to the solutions obtained in both a pure free-surface and a pure shallow-water simulation. Finally, a performance test is carried out to demonstrate that the overhead of the coupling is negligible. A quantitative validation of this type of coupling for the lattice Boltzmann method is novel, and opens the door to a range of large-scale simulations of canals and other hydrodynamic systems.