I've found that if the NPD starts feeling fear that they are about to be abandoned then they can quickly morph into the more classic BPD traits, like stalking, to show they care LOL.

As in the NPD cannot express that they care but they can do it secretly by their actions of stalking and remaining hidden, after they have probably driven the BPD over the edge. And vice versa. This situation gets reversed of course, where the BPD becomes the NPD and then when things are really at the end they both become NPDBPDWXYZ and it's just a warzone to see who can take each other out first. Who can stay away, who can look like they're staying away, who can stop talking about the other one to friends first, who can show they have moved on at the speed of sound.