A co-cathedral is a cathedral church which shares the function of being a bishop's seat, or cathedra, with another cathedral, often in another city. Instances of this occurred in England before the Protestant Reformation in the dioceses of 'Bath and Wells', and of 'Coventry and Lichfield'. These two dioceses were each named for both cities that served as bishop's seats.

The Archdiocese of Mechelen–Brussels is a Latin Church ecclesiastical territory or archdiocese of the Catholic Church in Belgium. It is the primatial see of Belgium and the centre of the ecclesiastical province governed by the Archbishop of Mechelen–Brussels, which covers the whole of Belgium. It was formed in 1559 and the bishop has a seat in two cathedrals, St. Rumbold's Cathedral in Mechelen and the Cathedral of St. Michael and St. Gudula in Brussels. The current archbishop is Luc Terlinden, who was installed in September 2023.