I was diagnosed with bipolar disorder and DID many years ago. The dx's have held up through so many providers I can't even count them all. And...yeah... treatment is complicated (to say the least).
Basically and very simplified, BD is neurological, while dissociative disorders are trauma-based. At least, that's the word on the street.
It's true that discerning between the 2 disorders can be tricky...sort-of. Both can include the memory loss spells. That said, BD and DID feel very different to me inside my mind and body.
It really is necessary to have professional guidance with these disorders - especially when they present together. The trick is hooking up with a professional(s) who know how to treat dissociate disorders. I've seen 30+ pdocs in my life and haven't yet found one that really gets dissociation - especially derealization and DID (they seem to comprehend depersonalization better; but they also seem to categorize dp as a psychotic symptom, which is only sometimes correct).
Trauma therapists, however, are (or should be) well-prepared to treat the full range of dissociative disorders.