Dual relationships can also be useful in alleviating pressure in one primary relationship that would arise if the person put all their eggs in one basket, as it were. That is one reason why there are often multiple hidden people coming and going in the intimate lives of pwBPD. In a strange way one r/s or potential r/s can almost enhance their ability to engage in another r/ship.
Having a relationship with 2 people would create a triangle.
Triangulation can shift focus off of the primary relationship, if they aren't coping, feeling engulfed. My uBPD was going through a disassociative phase yet trying to maintain two r/s's. After she left, she still acted as if nothing was happening or serious with the replacement, but was still trying to keep me at arms length to potentially fall back on.