Game concept
Co-Rulership
The game also links it as Co-Rulerships, Co-Ruler, Co-Rulers.
From the game's encyclopedia
Co-Rulers are a form of Power Sharing where the Diarch is considered to officially and permanently share the rule of the Realm.
They are always the Designated Heir (though may still lose elections), and can only be removed from office by Execution — unless their Liege has an appropriate Cultural Tradition, in which case they can be maimed instead.
Uniquely, when a Co-Ruler executes their Liege or defeats them in a war, they Usurp their Titles. In Cultures where maiming is an acceptable way to remove a Co-Ruler, then the Co-Ruler may also maim their Liege to Usurp them.
Examples of Co-Rulerships include:
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Co-Rulership is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Co-Rulers are a form of Power Sharing where the Diarch is considered to officially and permanently share the rule of the Realm." It is part of Diarch. It covers Duumvirate, Nominal Duumvirate and Co-Monarchy. The game also links it as Co-Rulerships, Co-Ruler and Co-Rulers. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.