Either is grammatical, according to the Cambridge Grammar of the English Language (Huddleston and Pullum 2002).
The comparative determinatives more, less, fewer can occur after the head provided there is a determiner. Thus we have a post-head alternant for one more day but not for more days: compare [One day more]/*[Days more] will be needed.
"Two items fewer" is ungrammatical because fewer refers to the number of items, rather than the items themselves. Therefore, "two fewer items" is correct.
two fewer items
is correct, meaning the count of my items is two lower than the count of theirs.