Interesting.
That's more like mating behavior instead of combat. I have witnessed both in the wild,, and entwining the bodies is a mating behavior,, while striking an opponent is combat. Yet,, we have 2 different species here that normally would not mate.
Maybe the female (likely the copperhead, hard to identify) is in heat,, while the cottonmouth is a male,, and doesn't care what species she is. Still very interesting, and unusual.