From what I understand, Borderline Personality Disorder doesn't tend to acutally "split" into separate (full-blown) identities. DID patients tend to have fully formed identities; with names, separate personailities and habits. And often a DID person's identities don't know what each other are doing, resulting in "blank spaces".
BPD has identity disturbance, meaning (I think) that you feel "unreal" at times, kind of like watching yourself doing things, and wanting to stop, but not being able to. But there aren't actual separate identities.
I know even with [just] bi-polar, I kind of feel that sort of dissociation. Where I know I'm reacting to things in an odd way, but can't seem to control what I'm doing. Like laughing hysterically, talking 90 miles an hour, or exploding over little things. But BPD sounds like a more severe version of this.