This is how I see it.
They come, even when included in the same cluster on the DSM, as different personality disorders for a reason.

In my opinion and from what I’ve read, the core of BPD is fear of abandonment while in NPD is having this self-centred identity built putting at risk by someone else or simply circumstances in life.
From that, I see both PD’s so different.
Someone with BPD needs others to complete their damage sense of self whereas someone with NPD renounced from the beginning to contemplate others as an equal. They don’t need anybody else. Maybe, covert ones, look for some kind of supply but other than that, they don’t consider the value of other people. In the world exist, them in the center and the others orbiting around them.

Then, they may display some openly kind of similarities (rage, thoughts distortions, etc)
By far, BPD is the richest PD in what symptoms display regards. And it’s to a point normal that there will be some similarities but only on the surface.
Example: Polarised thinking.
Someone with BPD: You love me or you hate me.
Someone with NPD: You are for or against me.