Can a "fight" between two witches end?
From the Minecraft Wiki under the trivia section:
If a witch hits another witch with a potion, the witch who is hit will become angry and target the other witch, however it is impossible for either witch to kill the other (without help from a player) due to their use of healing potions and their resistance to each other's splash potions
So from your comment, you kind of hinted at this. It looks like the healing potions make it impossible for either witch to die from one another and end the fight. Unless you intervene, they will continue to fight.