-- 1. Full Outer Join
SELECT NVL(a.id, b.id) id
     , a.cnt cnt_1
     , b.cnt cnt_2
  FROM (SELECT id, COUNT(*) cnt FROM t1 GROUP BY id) a
  FULL OUTER JOIN
       (SELECT id, COUNT(*) cnt FROM t2 GROUP BY id) b
    ON a.id = b.id
 WHERE NVL(a.cnt, 0) != NVL(b.cnt, 0)
 ORDER BY id
;
-- 2. Union
SELECT id
     , SUM(a) cnt1
     , SUM(b) cnt2
  FROM (SELECT id, 1 a, 0 b FROM t1
         UNION ALL
        SELECT id, 0 a, 1 b FROM t2
        )
 GROUP BY id
 HAVING SUM(a) != SUM(b)
 ORDER BY id
;