-- 기존 쿼리 살짝 고쳐도 되고...
SELECT MAX(gongj_seq)
  FROM mftb010tb
 WHERE plan_yymm   = is_plan_yymm
   AND order_no    = ls_order_no
   AND order_ln_no = ls_order_ln_no
   AND gongj_seq   < (SELECT MAX(gongj_seq)
                        FROM mftb010tb
                       WHERE plan_yymm   = is_plan_yymm
                         AND order_no    = ls_order_no
                         AND order_ln_no = ls_order_ln_no
                      )
;

-- 정렬과 ROWNUM 조건을 이용해도 되고...
SELECT MIN(gongj_seq)
  FROM (SELECT gongj_seq
          FROM mftb010tb
         WHERE plan_yymm   = is_plan_yymm
           AND order_no    = ls_order_no
           AND order_ln_no = ls_order_ln_no
         ORDER BY gongj_seq DESC
        )
 WHERE ROWNUM <= 2
;