OP, the truth is, this situation is just "normal" to me. I remember questioning why my grandfather didn't leave, but that was mybe 10 years ago the last time. They are old, they need each other, and I don't haveto live there. That said, I don't tolerate listening to them snipe at each other. When it gets started, I just leave. I love them both and won't listen to them tear each other down. So, I guess I set my own boundaries with them then took a posture of nuetrality. I don't know if that would be at all helpful for you though.
Me - Mostly mono female, 39 yrs old, married to Leo.
Leo - Poly bi-sexual male, 37 years old. Married to me and looking.