I think that's how English nobility worked/works, too, not sure though.
It's... complicated. The Royal House is Windsor, which is the legal surname of the ruling monarch, but the surname used by the Queen's descendants is Mountbatten-Windsor, combining both her name and that of her husbands. Sounds simple, but it actually needed to be figured out before their marriage and later in a court to work out due to all the complicated matters of who precisely got what surname and titles etc.
And I love the idea that Cullen's a magespawn, headcanon accepted!
His mother was a mage and his father smelled of elderberries! 