From what I've learned in graduate school, both disorders are so misunderstood that it is common for bipolar to be mistaken for BPD and vice versa. That's not to say they can't co-occur, it's just that having a diagnosis of one does not make you more likely to have the other. BPD is a personality disorder brought on by childhood trauma, where bipolar disorder has more of a genetic basis and occurs with or without childhood trauma.