We have measured thermally stimulated luminescence (TSL) in tri-p-tolylamine doped poly(styrene) containing various concentrations of different extrinsic shallow traps. The evolution of the distribution of localized states in these doubly-doped polymers was studied with increasing trap concentration and trap depth. We show that the observed trends in the TSL can be reasonably well understood, in terms of the Gaussian disorder model, as thermal release of charge carriers from the tail states of the density-of-states distribution. Our approach is based on analysis of the fractional TSL data and allows extraction of a parameter characterizing the energetic disorder in disordered organic solids. The effect of shallow extrinsic trapping on TSL properties, due to molecules with lower ionization potential, can be reasonably interpreted in terms of the effective energetic disorder and the TSL results are in good agreement with those obtained earlier by charge transport studies.