Aeque principaliter ("equally important") is a Latin term used by the Roman Catholic Church to indicate a merger of two or more dioceses in which – to avoid questions of predominance – the dioceses are all given equal importance. Such a merger often followed a merger in persona episcopi.
This type of union essentially consists into the fusion of two or more circumscriptions into only one. As a consequence, this new diocese will have two or more episcopal sees and cathedrals, which correspond to those of the previous dioceses.
To give an example, the Roman Catholic Archdiocese of Pamplona and Tudela is a single diocese established by the union aeque principaliter of the Archdiocese of Pamplona and the Diocese of Tudela.
Through this fusion, the new circumscription has two episcopal sees: Pamplona and Tudela.
As a consequence it has also two cathedrals, one for each episcopal see.