It's very possible to have both. They do mimic each other, really. BPD also has some major relationship aspects to it that aren't necessarilyi there with bipolar.

One of the best things a person with BPD ever said to me to help me get the difference is that with her, she lives an "I love you, I hate you, don't leave me!" life. So it's like being super clingy while pushing people away at the same time, and this inability to relate properly in relationships causes her to have dramatic mood shifts.

My psychologist had talked to me a bit about the possability of being BPD, because I do have some abandoment issues, due to my mom dying when I was a kid. But, the reason I lost my friends was because they were the ones who were toxic and left me, I didn't cut them out, they cut me out. My first marriage failed most likely due to me having gotten married while manic to begin with. And, my relationship with my husband is as stable as an untreated person with bipolar and one with untreated ADD person can be.

Try taking this quiz and see how the results come out:
http://psychcentral.com/quizzes/borderline.htm