It has already been determined, some time ago, that where there are two flats, both must be served. To allow otherwise undermines the overriding purpose of the legislation to grant the R2FR.
Both do have the choice to exercise the right together, the legislation does not start at "only 3 flats or more" may qualify.
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers.