Elevate Co-Monarch

is_available = trigger_if = {
	limit = {
		is_ai = yes
	}
	age >= 50
	may_appoint_co_monarchs_trigger = yes
	any_child = {
		is_adult = yes
	}
	has_active_diarchy = no
}
is_shown = scope:actor = {
	may_appoint_co_monarchs_trigger = yes
}
OR = {
	scope:recipient ?= scope:actor
	scope:secondary_recipient ?= {
		is_child_of = scope:actor
	}
}

As written in game/common/character_interactions/00_diarch_interactions.txt.

scope:actor = {
	has_active_diarchy = no
}
trigger_if = {
	limit = {
		exists = scope:secondary_recipient
	}
	scope:secondary_recipient = {
		valid_co_monarch_candidate_trigger = {
			LIEGE = scope:actor
		}
	}
}
scope:actor = {
	is_available_at_peace = yes
}

As written in game/common/character_interactions/00_diarch_interactions.txt.

on_accept = scope:actor = {
	if = {
		limit = {
			NOT = {
				exists = scope:ai_recipient
			}
		}
		custom_tooltip = {
			text = appoint_co_monarch_interaction.tt.enter_co_monarchy_with_secondary_recipient
			try_start_diarchy = co_monarchy
			set_diarch = scope:secondary_recipient
		}
		designated_heir ?= {
			if = {
				limit = {
					this != scope:secondary_recipient
				}
				custom_tooltip = appoint_co_monarch_interaction.tt.overwrite_designated_heir
				save_scope_as = old_heir
			}
		}
		set_designated_heir = scope:secondary_recipient
	}
	else = {
		try_start_diarchy = co_emperorship
		set_diarch = scope:ai_recipient
		designated_heir ?= {
			if = {
				limit = {
					this != scope:secondary_recipient
				}
				save_scope_as = old_heir
			}
		}
		set_designated_heir = scope:ai_recipient
	}
}
grateful_for_appointment_as_co_emperor_effect = yes
diarch_overwrites_old_designated_heir_apply_opinions_effect = {
	LIEGE = scope:actor
	CO_RULER = scope:secondary_recipient
}
designated_diarch_overwridden_by_co_ruler_effect = {
	LIEGE = scope:actor
	CO_RULER = scope:secondary_recipient
}
if = {
	limit = {
		scope:secondary_recipient = {
			is_ruler = no
		}
		scope:secondary_recipient = {
			NOT = {
				is_in_the_same_court_as = scope:actor
			}
		}
	}
	scope:actor = {
		add_courtier = scope:secondary_recipient
	}
}
if = {
	limit = {
		scope:secondary_recipient.liege != scope:actor
	}
	scope:secondary_recipient.liege = {
		add_opinion = {
			target = scope:actor
			modifier = retracted_vassal
		}
	}
	create_title_and_vassal_change = {
		type = revoked
		save_scope_as = change
	}
	scope:secondary_recipient = {
		change_liege = {
			liege = scope:actor
			change = scope:change
		}
	}
	resolve_title_and_vassal_change = scope:change
}
scope:actor = {
	diarch_interaction_set_or_update_global_variable_effect = {
		VARIABLE = di_track_appcomon_accept
	}
	diarch_interaction_add_to_global_list_effect = {
		VARIABLE_LIST = di_track_appcomon_accept_actor_list
	}
}
scope:recipient = {
	diarch_interaction_add_to_global_list_effect = {
		VARIABLE_LIST = di_track_appcomon_accept_recipient_list
	}
}

As written in game/common/character_interactions/00_diarch_interactions.txt.