You can have both Bipolar and BPD. I do, and now that I am diagnosed with both. I can definitely see the difference between the two, and how they play off of each other.

The key (for me) is when the Bipolar is acting up is that more often it is without a direct trigger, and the moods are persistent. The depression is just there, it doesn't really go away no matter what happens. Also,the hypomania that I experience comes with distinct physical sensations of energy.

The BPD comes in a chain. Just as an example "You hurt me. I must be horrible. I hate myself." and then I get depressed.

They do come up at the same time, and things can get confusing. Thank goodness DBT works for me, now I can learn to sort out the behavior chains from the symptoms that need medical attention.