Part of Ongoing Effect
If you play King of the Realm and then I play King of the Realm afterward, the game applies both control effects in layer 3. Since your King is older, the game starts with that effect. Your King gives you control of all mortals, including my King. Then, when the second King's ability is being applied, he's already controlled by you due to the first King's ability. Therefore, you control all mortals and the first King 'wins.'