I think the two disorders are vastly different, but that doesn't prevent too many providers from mixing the two up. Though some say that some T's and pdocs prefer not to diagnose BPD because of the stigma, and will then diagnose Bipolar Disorder instead. Frankly, I don't know how common this is.

The core criteria for those with BPD is fear of abandonment. This can cause a whole host of interpersonal issues. And here we come to 'mood.' Those with BPD will have their mood enormously affected by interpersonal issues, even if it's for example, someone not returning your call, or not as soon as you would like. Some push people away in order to not get abandoned and then will do anything and everything to get that person back once they have decided they've had enough.

Bipolar Disorder is not a disorder of interpersonal relationships, though we can have problems with relationships just like anyone else can. Depression and/or hypo/mania can be produced by seemingly nothing at all, or lack of sleep, or stress, etc. Sometimes interpersonal relationships can be a source of stress, and therefore could lead to an episode, but you wouldn't have this push-pull, I love you-I hate you issue. Some people with BPD can also be volatile when it comes to anger and others feel like they have to walk on eggshells to not trigger them.

Also, those with Bipolar Disorder have episodes that are persistent, not fleeting: they will last for weeks, not a day, or less. They don't pinball between one mood and another. In fact, I wouldn't describe them so much as 'moods' as episodes; it is an episodic disorder whose episodes are generally not, unlike with BPD, triggered by interpersonal issues.

I suspect it's more common for those with Bipolar Disorder to have some traits of BPD, than to meet the full criteria. I think it's unusual for people to have full-blown BPD and Bipolar Disorder together, but that's just my opinion. People can have all kinds of co-morbid disorders, but BPD and BP are so very different, it's just hard for me to imagine it being common to have both.