I think that when pdocs, or when people self-diagnose, decide the diagnosis purely based on mood lability, you can get into a gray area, especially if one accepts the existence in BP of ultra-rapid cycling.

I think that when pdocs diagnose only on that basis, they can make serious mistakes. You have to take into account other characteristics, such as fear of abandonment in the case of BPD, and the extended length (usually) of episodes in the case of BP, to make an accurate diagnoses.

So I do believe the two can coexist, but I think there's a lot of faulty diagnoses out there as well.