The Equestrian Diarchy: (Head Canon Speculaions Included) · 10:37pm Feb 9th, 2015
Since Celestia and Luna are ruling the nation equally, not as King and Queen, or in this case, prince and Princess, that makes it into a Diarchy, as opposed to a Monarchy.
When we learn of Cadence and later Twilight Sparkle as princesses, this doesn't change the overall Government, it is still Celestia and Luna on the Royal Throne.