This is another article in the chapter Justice from the classical manual on Sharia called Reliance of the Traveller. The articles are posted one by one in the order they are given in the book.
THE INVALIDITY OF A PLURALITY OF CALIPHS
025.6 (K: It is not permissible for two or more individuals to be invested with the caliphate (H: at one time), even when they are in different regions, or remote from one another, because of the disunity of purpose and political dissolution it entails. If two are simultaneously invested as caliph, neither’s caliphate is valid. If invested serially, the caliphate of the first of them is legally valid and the second is disciplined (def: 017) for ommitting an unlawful act, together with those who swear fealty to him, if they are aware of the first’s investiture as caliph) (Mughni al-muhtaj ila ma’rifa ma’ani alfaz al-Minhaj (y73), 4.132, and Hawashi al-Shaykh ‘Abd aI-Hamid aI-Sharwani wa al-Shaykh Ahmad ibn Qasim al-‘Abbadi ‘ala Tuhfa al-muhtaj bisharh al-Minhaj (y2), 9.77-78).