RNRB shared ownership

When, if at all, is the RNRB lost of property is divided up. i.e. property is 100% owned by couple value £1.2 m. If the property was split into four with two children would the parent lose the RNRB?
Folioing on when is the RNRB lost on division of ownership? Could parents go down to say a 10% ownership and retain the RNRB?