SELECT a.lv
     , b.columna
  FROM (SELECT LEVEL lv FROM dual CONNECT BY LEVEL <= 2) a
     , (SELECT ROW_NUMBER() OVER(ORDER BY columna) rn
             , columna
          FROM t
         GROUP BY columna
        ) b
 WERER a.lv = b.rn(+)
   AND b.rn(+) <= 2
;