Row_Number() con consulta de unión

Prueba esto:

SELECT *, ROW_NUMBER() OVER(ORDER BY Id) ROW_NUM
  FROM (
        select Id, VersionNumber from documents where id=5 
        Union all  
        select Id, VersionNumber from versions where id=5
       ) a
order by VersionNumber desc

PARA filtrar por número de versión 5 use:

SELECT * 
FROM   (SELECT *, 
               Row_number() OVER(ORDER BY versionnumber DESC, id) row_num 
        FROM   (SELECT id, 
                       versionnumber 
                FROM   documents 
                WHERE  id = 5 
                UNION ALL 
                SELECT id, 
                       versionnumber 
                FROM   versions 
                WHERE  id = 5) a) b 
WHERE  version = 5