Ruling Couple: The Imperial Couple are – as really, you should have been able to guess from the name – this trope.  Diarchies in which the co-rulers have equal authority and mutual veto was the governing tradition of Cestia long into pre-Imperial history (in fact, most Cestian political offices worked this way), was continued into the Union of Empires as a way to emphasize the equal status of Alphas I of Cestia and Seledië III of Selenaria in that, and then, due to demonstrably providing welcome redundancy, versatility, and an additional check and balance to the executive branch, found themselves written into the Imperial Charter as they were.