this command makes 2 entity's fight (unless they are passive mobs) also make sure you name the mobs with an anvil, /damage @e[name=Example] 0 entity_attack entity @e[name=Example]
P.S: 1. The rivalry is friendly, not really getting angry, just trying to outplay eachother 2. I did try to make a rivalry on another RU-vidr called ThatOnegamerX, but he ghosted me, most likely because he's been inactive for over 6 years. 3. fun