Like the well known perovskites, the double perovskites A2BB'O6 form an enormous family of compounds exhibiting a wealth of phenomena ranging from magnetic long range order, multiferroicity, spin liquid behaviour or ferroelectricity. Recently, the vital role of spin orbit coupling in determining their ground state properties has been realised, particularly in the higher 4d and 5d transition metal systems. We propose an inelastic neutron scattering investigation of the excitation spectrum in two model osmium based 5d1 and 5d2 double perovskites, model systems for which comprehensive theoretical predictions have been made, but where detailed Q resolved studies of the dispersion are lacking to date