Hey HD, I know it's been a while since you posted this, but as someone who has a bit of insight I just thought I'd get this down anyway.
Basically, you can view BPD and DPD as kind of polar opposites yet complimentary PDs. As someone who has dependent tendencies, I found myself in quite a few romantic and platonic relationships with people with BPD. The reason for this is that as a Dependant, my whole existence depends upon me being dependended upon of sorts. By getting into relationships with people who have BPD, I felt as though I was there to help them and I could break through the problems and they would value me and need me as a result. Of course, this never ended well as dysfunctional can't really fix dysfunctional.
So, in a nutshell, us dependents depend on others depending on us. If that makes any sense. Lol. It took me a while to get my head around this, and I'd always wonder why I fell into the same patterns with the same dysfunctional, needy people. Turns out I was just as needy in a submissive kind of way.
Hope this helps.