Shock tube experiments are performed in order to study shock propagation along a two-gas layer in a confined geometry and to compare it to the case of a homogeneous density equivalent mixture. The analysis of the homogeneous case gives values for the adiabatic coefficient and density of the mixture of both gases, while the comparison between heterogeneous and homogeneous media with the same averaged density shows modifications of the shock front shape and velocity. In the two-gas layer, the shock propagates faster than in the homogeneous medium. The shock front is curved with a triple point which appears close to the shock-tube wall, in the slow medium, while it stays planar during its whole propagation in the homogeneous mixture. A correlation is found between the angle of curvature and the shock velocity increase. It is confirmed by two-dimensional Eulerian numerical calculations. Experiments and calculations exhibit very good agreement on all the measurements when molecular diffusion is taken into account in the numerical calculations. A sustained irregular refraction pattern of the shock front at the diffuse interface of both gases is obtained experimentally and confirmed by the calculations.