SELECT DISTINCT T.A, T.B
FROM T
ORDER BY T.A, T.B
FETCH FIRST 2 ROWS ONLY

Теперь проблема в сортировке строк.
Придется завести еще одно поле для указания порядка (Q).

WITH Q1(Q, A, B, C) AS
(VALUES(1, 1, 1 ,'xxx'),
(2, 1, 1, 'eee'),
(3, 1, 1, 'ddd'),
(4, 2, 1, 'fff'),
(5, 3, 4, 'ddd'),
(6, 3, 4, 'dfdd'),
(7, 3, 4, 'fff'),
(8, 3, 5, 'kkk')),

Q2(Q, A, B, C) AS
(SELECT Q1.Q, Q1.A, Q1.B, COUNT(Q2.Q)
FROM Q1
INNER JOIN Q1 Q2 ON (Q1.Q >= Q2.Q AND Q1.A = Q2.A AND Q1.B = Q2.B)
GROUP BY Q1.Q, Q1.A, Q1.B
HAVING COUNT(Q2.Q) <= 2)

SELECT Q1.*
FROM Q1
INNER JOIN Q2 ON (Q2.Q = Q1.Q)