Du har brug for privilegiet CREATED ANY SYNONYM for at gøre det som A, derfor
GRANT CREATE ANY SYNONYM TO A;
EDIT:For at undgå ENHVER privilegium skal du gøre dette:
a) som A:
GRANT SELECT ON mytable1 TO B;
GRANT SELECT, INSERT, UPDATE, DELETE ON mytable2 TO B;
b) som B:
CREATE SYNONYM a_mytable1 FOR A.mytable1;
CREATE SYNONYM a_mytable2 FOR A.mytable2;