I dare to say that BD cycles and are biologically organic in nature, where as BPD cycles are triggered because it's a personality condition caused by early trauma and abuse in the life of a child. Early abuse is a precursor to the BPD classification.

Once a person has BPD, a dissociative disorder can develop through unprocessed trauma. This can lead to secondary structural dissociation, a far distant cousin to DID. This is a condition where there is one distinct state (apparently normal part (ANP)) and 2 or more indistinct state (emotional parts (EP)). The EP doesn't take over consciousness.

DID is a tiertary structural dissociation where a person has 2 or more ANP and 2 or more EP. The ANP are the states that take over consciousness, per DSM-5.

BD is treated meds first followed by therapy whereas BPD is treated by therapy first followed by meds to control the EP switching.

A person can have both conditions. Some of my alters are BPD.

I hope this helps, I've got issues and I've been doing lots of reading.