I was just wondering out of curiousity, what is the nature of a relationship when BOTH parties are BPD?
I don't think, that there is an exact answer to this question. Borderline is a very complex disorder which lies on a more complex spectrum with other PDs. Someone can be BPD with traits of HPD, NPD, ASPD or even DPD (which is not in the Cluster-B). We are talking about human beings, not about machines. A pwBPD who as also ASPD-traits will dominate a pwBPD who has DPD-traits, I think.