After learning about bpd and narcissism I learned that a bpd and a narcissist can have a functional romantic relationship as long as the narcissist constantly showers the bpd in love even though it will only occasionally be returned and they will have high abandonment anxiety (which is why so much unconditional love is required). The narcissist has to remember that their partner is the best in the world because they're the best in the world (the narc is) and so they would only pick a good partner. As long as the narc never enters the devaluation phase I think it should work.