You mean search for images that have KeywordA but not KeywordB? Yes, that is possible. If you add an exclamation mark before a word, it means 'not this word', so in the filter bar you can search for 'Keywords contains words KeywordA !KeywordB'. You can also create a smart collection with two lines:
Keywords contains word 'KeywordA'
Keywords does not contain 'KeywordB'.
-- Johan W. Elzenga