In the wake of recent analytical work highlighting the role as synchrotron emission sources of both the forward and reverse shock emerging from the collision of two ultra relativistic shells, we perform a numerical study of such a system and its observed flux. We precise the hydrodynamical structure, properties, and resulting emission timings by extending the approach to spherical geometry. We confirm the reproduction of GRB spectral features explained by the simpler analytical approach such as the sub-dominant low-energy spectral component and the doubly-broken power-law spectrum without any fine-tuning of the physical conditions, and lay the foundation to explore emission from internal shocks of more complex outflows.