My abuser dated two of us at the same time, and he seemed to be pitting us against each other intentionally. Purposefully making us jealous, telling one of us the other had said something horrible about them, banning one of us from being talked about and talking constantly about the other, sparking situations where we would fight.
It felt like we were supposed to compete to be the best partner or something, and one of us would always be getting left behind. In the end, we both left him.