I am the emperor of Hispania and Francia and have two vassal kings, the king of Navarra and the king of France. The king of Navarra doesn't desire the empire of Hispania, but the king of France has a -20 opinion penalty because he desires the empire of Francia. Why isn't the effect symmetrical? Neither one is ambitious/envious.
Your vassals will never "desire" your primary title. Presumably, Emperor of Hispania is your primary title, while Emperor of Francia is a secondary one. The King of Navarra would then not have the penalty, while the King of France would. (Source)