Narcissism splitting vs Borderline Personality Disorder Splitting. How can you know the difference between Npd and Bpd splitting?

I've been seeing a guy for 6 months. He has both traits of Npd and Bpd. I know they can overlap. I have been reading about splitting in both disorders. How can i determine which he is?

Narcissism splitting vs Borderline Personality Disorder Splitting. How can you know the difference between Npd and Bpd splitting?
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