Get the rows where id_user_rq exists in id_user_ap and viceversa
Please try this:

SELECT A.* FROM
(SELECT * FROM default_relation_users WHERE id_user_rq = 1) A
LEFT JOIN
(SELECT * FROM default_relation_users WHERE id_user_ap = 1) B
ON
(A.id_user_rq=B.id_user_ap AND B.id_user_rq=A.id_user_ap)
WHERE B.id_user_ap IS NOT NULL;