The notion of "parallel sovereignty"[1] between premodern States and the Catholic Church was an accepted situation in medieval and early modern Europe to such an extent that it was considered as the ADN of the occidental society.
[2] Even if this authorities were often in conflicts, Church and State were habitually supportive for each other et it's possible to called it a "legal collaborative pluralism".
[8] Over time, those distinctions tended to break down, and individuals would choose to bring their legal claims under the system that they thought would offer them the best advantage.
Sources of Islamic law include the Koran, Sunnah and Ijma, but most modern Western nation-states take the basis of their legal system from the Christian superpowers of old (Britain, France etc.).
In Australia, for example, the Mabo decision gave recognition to native title and thus elements of traditional Aboriginal law.
As a consequence, members of the Committee on the Elimination of Discrimination against Women (CEDAW) have called for a unification of legal systems within countries.