Employment is not covered employment if performed: for an employer that is determined to be subject to the Railroad Unemployment Insurance Act by a unit authorized by federal law to make that determination; or as an employee representative who is determined to be subject to the Railroad Unemployment Insurance Act by the unit. If the Railroad Retirement Board determines that a person is engaged principally in a business other than the carrier business, the exclusion from insurance under paragraph (1) of this subsection applies only to employment for the identifiable and separable enterprise of the person that the Secretary determines to be considered the employer. Employment is not covered employment if performed for an employer that is determined by a unit of the federal government authorized to make the determination to be subject to any other unemployment insurance system established by federal law.