The biggest #1 difference is that bipolar is a chemical imbalance in the brain that can almost always be cured or treated successfully with drugs. It sometimes takes a while to discern which drugs, but once those are found the difference can be night and day for the sufferer. Borderline is believed to be almost 100% learned. Drugs may sometimes be used to take the edge off to help people cope, but drugs cannot cure it as far as we know right now.

Also, a BPD person can cycle insanely fast. They can feel a half dozen distinct, strong emotions in just a minute or two. Bipolar, even fast cycling ones, don't do that. And then there's the relationship and abandonment issues that most BPD have. I don't believe bipolar have those as well. I'm not an expert on bipolar, however, and it may be better to talk to someone who is.

And yes, it is possible to have both. I have at least one friend on here who fits that description.