I agree, but I think sacredness likes to invade sanctity's territory, as in "sacredness of life".Not a lot. 'Sanctity' is more latinate, using the '-ity' suffix (fom -itia) On the other hand, 'sacredness' uses the more home-grown '-ness' suffix. O|therwise, there is a lot of overlap.
However, the root adjectives (from which the abstract noun is derived) are different in meaning:
sanctus (Latin) - holy, that should be regarded as special in some way, having what some people would regard as 'holiness'
sacred - set apart in some way, perhaps blessed in some church ritual ('consecrated')
Check out their collocations. I think you'll find that 'sanctity' usually refers to an intrinsic quality (e.g. 'the sanctity of human life'), while 'sacredness' refers to an assigned and accidental quality (e.g. 'the sacredness of Jerusalem to so many conflicting faith groups').
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