It seems that an alternative air speed measurement is needed to augment the pitot tubes, something that would be affected differently by environmental effects and other failure modes. I'm not suggesting replacing pitot tubes, because that would just introduce a different, but still single, failure mode, rather than introducing redundancy. What comes to mind is Doppler measurement of back-scattered energy, either EM or acoustic. A quick search reveals that an optical version of this is already being developed in the UK (though they seem to want to replace pitot tubes):
Emerald: Article Request - New optical airspeed sensor poised to cut airline costs
Likely other groups are working on this also.