if an English mastiff and a bull mastiff have puppies can you register them as doubull mastiffs?

No, you cannot register puppies from an English Mastiff and a Bull Mastiff as "Doubull Mastiffs," as this term is not officially recognized by major kennel clubs. The puppies would typically be classified according to the breeds of their parents. If you intend to register them, they would most likely be registered as mixed breed or under the respective breed categories, depending on the rules of the registering organization.