I believe I am reasonably qualified to express myself on the topic. I am a recovering codependent who went to my first Codependents Anonymous meeting in 1990. And a recovering, principally impulsive-type borderline since 1997. As well as the adult child of a severely petulant-type borderline... and a former chaser of them into bumpy-road romance for decades.
I'd had no idea whatsoever about the reciprocal reactivity "fur ball" until I stumbled onto Harry Stack Sullivan's notion of "parataxic integration" while I was (again) in school in 2004 reading Sullivan student Lorna Smith Benjamin's Interpersonal Diagnosis and Treatment of Personality Disorders.
Though I would love to say I connected all the dots together in seconds, it would be thoroughly erroneous to do so. But I was playing emotional dodge ball with yet another borderline on the other side of my computer screen a year or so ago when I began to at least get the dots closer together.
Now. Here's where we need to take a sudden left turn, insist upon a "prerequisite," and get explanatory. So please see this article so you understand at least some of what I have come to understand after having dealt with well over a hundred borderlines since 1987.
Got all that? Okay, good. Because we -- as borderlines -- are a) reciprocal reactors, as well as b) hyper-codependent and attachment-seeking here and hyper-counter-dependent and attachment-rejecting there. Why? because we grew up conditioned, in-doctrine-ated, instructed, socialized, habituated, normalized and neurally “hard-wired” to be desperate for connection here and desperate for protection there.
(See Why do we get so Desperate for Connection? An Answer from the Purview of Attachment, Early Life Research & Codependency as well as the "protector" in the Internal Family Systems Model.)
Which makes us suckers for seduction -- as well as seducers ourselves -- and addiction-prone, hair-trigger reactors to even the slightest hint that the seducers we reciprocally react to by seducing them have turned ("suddenly") into the same sort of vile abusers we were raised by. Which was all too often a mother who baited and then bit, showing us (however subconsciously) how to seduce and abuse. (I assert the "bad mother" along with many of the early experts on BPD like Winnicott, Sullivan, Mahler and Klein because the "dark diagnosis" is so often the result of some combination of having been neglected, ignored, abandoned, discounted, disclaimed, and rejected, as well as invalidated, confused, betrayed, insulted, criticized, judged, blamed, shamed, ridiculed, embarrassed, humiliated, denigrated, derogated, set up to screw up, victimized, demonized, persecuted, picked on, vilified, dumped on, bullied, gaslighted, scapegoated, emotionally blackmailed and/or otherwise abused by others upon whom they depended for survival in the first few years of life. Producing a nasty and lingering case of sometimes utterly intolerable Complex Post Traumatic Stress Disorder.
(Can a "bad dad" cause CPTSD > BPD? Of course, but the character of BPD seen in adolescence and adulthood tends to look different: It's more compensatory narcissistic and less like Learned Helplessness & the Victim Identity. In my opinion, that is because "bad dad" BPD usually takes place at a later stage of development on Piaget's, Kohlberg's and Erikson's developmental paths when a child has somewhat more evolved cognitive capacities, understanding of morality and autonomy, initiative and competence, respectively.)
Most of the self-proclaimed "codependents" I have encountered over the past 30 years exhibit mild to moderate BPD traits though those traits will look a lot different "up front" and "at first" if the person is a petulant or impulsive vs. self-destructive or discouraged type of borderline. (Ultimately, all four "types" will show up, but usually not for a while.)
Moreover, codependents who have moderate to florid BPD traits find it extremely difficult to see, hear, feel or sense themselves because of their Dissociation into various Internal Family Systems Model "alters." (Some readers will jump up and down here about supposedly absolute demarcations between BPD and DID, and they're welcome to do so. But I've studied Kluft, Putman, Herman, Courtois, Lynn & Rhue, van der Kolk and van der Hart in depth; and I have known a LOT of people with DID and BPD. The demarcation is anything but absolute or rigid, is actually quite plastic and varies considerably over time.)
Some alters can "self-see." Most cannot. And many get very hostile when they think they are being "forced" to do so out of understandable and unprocessed shame, guilt, worry, remorse, regret and anxiety, as well as overwhelming terror and reactive rage. And when in those states of gross autonomic agitation, they will act out according to being in any one (or more) of the Fight, Flight, Freeze, Faint, Feign (or Fawn), Freak and/or Fry responses.
Fortunately, this rarely occurs in CoDA meetings, but on occasion, it does. Especially when the borderline flips (as they do and they will) from an acquiescent, submissive, attachment-seeking identity to one that dimly recalls what that gawd-awful abuser did to them and sees a new version of that abuser in the meeting. (I never did that. Hah!)
NOW. Please disabuse yourself of any notions that you can use any of this information to edify or “manage” a troubled codependent borderline in extremis, let alone in the heat of the moment. The information here is supplied simply for recognition, acknowledgment, appreciation and understanding of what's going on in the long term... and management of one's reaction to a borderline's sudden rage.
Resources: See the section on Borderline Personality Disorder in A CPTSD Library.