I created the monarchy of my two planets similiar to this however they have and wield the power, though the one King does maintain a council while both make sure the populace is happy
The Kings and royalty are pro-country first before personal, the throne is also where the allegiance is directed, not to a person but to the throne.
As for the religion, the Kings, one more along agnostic while the other King is devoted. there is a religious hierarchy that runs the religion and not the royalty.
However in the book, either King or royalty can be prosecuted and dealt with for committing murder, treason, or belonging to an forbidden brotherhood.