I wouldn't exactly call it a "good" pairing, but it is a common pairing because the dysfunctions of the two PDs feed into each other heavily.

The only way it can possibly be remotely therapeutic is if both partners are actually trying to change for the better.

A BPD tends to have a very aggressive, righteous judgement for others, and typically a BPD rage involves going for the throat about what an awful person you are (real or imagined, usually a fair combination of both). However, the BPD's deep fear of abandonment usually means that the BPD will still cling to you and try to mend the relationship, even after they have torn you a new one explaining what a demon you are.

For the NPD partner, this means that their "false self" shield is going to be systematically obliterated, yet they will also experience being accepted even after their true self is violently dragged out into the open. So in theory I guess this could potentially be somewhat healing for an NPD.

Then NPDs are the masters of playing "keep away" and mind games, which means that the BPD with their martyrdom complex is eventually going to be driven to that explosive point of actually declaring what they want, what they need, how they feel and what pisses them off. Even if it comes out in the form of lots of yelling and swearing with a couple implied suicide threats thrown in for good measure. But in most cases ultimately the BPD will be pushed to a point of getting in touch with their suppressed emotions (which will likely come exploding out in volcanic glory) and actually communicating their needs. So again, in theory, it could possibly be therapeutic for the BPD, as well.

However it would certainly be a seriously rough and messy and possibly even dangerous form of "therapy" for both parties. But to each their own, of course.

This is my personal perception based on my experience as someone with strong BPD tendencies who tends to attract NPD partners.