Hello Heidi
Welcome to the HPD forum.
In my opinion, HPD seems like a more refined version of BPD. Like BPD's, HPD's get depressed, dissociate, and often have suffered some sort of significant psychological trauma as children, but beyond that, HPD behavior also seems to be the result of some sort of childhood training, in which the child was rewarded for being something than she (or he) fundamentally wasn't. Essentially the child acted out some theme that was appealing to the parents, other adult caretakers, or sometimes the entire family - in the process often reducing some kind of potentially disruptive tension within the family - and hence holding it together. Or, the child alone thought that the acting would bring her love from one or both parents. HPD's are often considered physically attractive, which in and of itself primes them for being utilized as child actors for the various needs of dysfunctional adults.
There have been several members on the HPD forum who said they have been DX'd with both BPD and HPD. In general, it seems to me like HPD has a strong (genetic?) connection to BPD, probably stronger than to NPD, and almost certainly stronger than to AsPD.
A researcher named Millon has described a subtype of HPD: Tempestuous, in which the affected person is prone to BPD-like emotional lability - outbursts...perhaps the 'blame storms' and other persecutory rants that some BPD folks have had to experience. So maybe Tempestuous HPD is a cross between HPD and BPD (?)
But although we like cautious, intelligent discussions about such matters, this is still only speculation. Professional psychotherapists and others trained in Personality Disorders see many, many disordered folks in the course of their careers. That is why we at PF always recommend that a person not diagnose themselves or others, and instead let a trained and competent professional give a person a final opinion of what's going on.
Beside the great folks you can interact with here (some of whom might be BPD X HPD), there is also the BPD forum, which can provide you with more information by those affected by BPD.
So again, welcome
Best,
orion