I am sorry you are miserable with them fighting.
Could choose to ask them to stop. Could choose to tell them what you expect from their behavior to stay in right relationship with you. Could be something like:
"I am not going to promise exclusivity. I want to be with you both, in a "V." Is this something you would be willing to participate in AND behave in civil fashion to your metamour, my other sweetie?
I do not expect you two to be best friends. I expect you to be civil and polite. I expect you to play nice. Because you care about me and don't want to hurt me.
When you compete and fight with my other sweetie over me like I am a prize, that treats me like a THING and not a person. That devalues and hurts me.
When you fight with my other sweetie you are hurting me by putting me in the middle. That causes me emotional and mental pain.
When you fight with my other sweetie, it's a drag, and doesn't make me looooove to hang out with you.
So please tell me if you are willing to participate in the creation of a harmonious "V" with me or not. If not willing, then please let's just not be romantic dating partners and just be friends instead so the bickering stops."
You asked for links: