A system of two parallel slab unidentical waveguides, one a multimode structure and the second a single-mode one, is analyzed using an improved coupled-mode theory. The theory is compared with exact numerical solutions of the complete structure. The comparison is carried out for both TE and TM modes, by evaluating the propagation constants and the normal mode distributions. The approximated propagation constants and the corresponding normal modes are shown to agree very well with the exact solutions. The interdependence between the system parameters and the normal mode patterns is discussed.