I think it's pretty common to end up with BPD partners, especially if you grew up in a family where there were PDs. I look back on the men I dated, and there is a lot of NPD/BPD behaviors. I divorced my son's father 3 years (N/BPDx) and finally put two and two together: the only variable that was constant in all these failed relationships was me. Three years of intense individual and group therapy later, I met someone healthy. I've been dating him for a year and a half. The key is to move really slowly! Know what you value, have excellent boundaries, assert yourself, and love who you are. Someone wonderful will feel the same way about yourself that you do.
But to your question about two BPD married to each other. I'm not familiar with that dynamic -- My dad has N traits and my mom is codependent, so that's my starting point. But there is a book review on bpdfamily about a book called the N/BPD Couple:
https://bpdfamily.com/message_board/index.php?topic=56788.0I'm also wondering if you might find adult children who grew up with this kind of PD duo over on the Healing and Coping boards? That might give you some idea about what their childhoods were like.