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You can not call vassals of another kingdom to war, they need to be independant. If you check your allies list, only the allies in green can be called to war.
The titles are not merged until you hier inherits both titles, she is still the duchess and vassal of the other kingdom. Her marriage to an outsider does not make her title leave that kingdom, infact if her liege has High CA or higher her title will not pass out of the kingdom.