SQL & PL/SQL

hello all,
i am facing some issue when i use with clause and union all operator.
i have created a dummy code to solve this problem..
my code is ----------
with dept_1 as
(select deptno d1 from detp9 where deptno = 20)
select empno from emp9 e,dept_d1 where e.empno = dept_1.d1
UNION ALL
with dept_1 as
(select deptno d2 from detp9 where deptno = 30)
select empno from emp9 e,dept_d2 where e.empno = dept_2.d2.

when i ran this i gort a message-
ORA-32034: unsupported use of WITH clause.
please help me to solve this iisue..
when i ran it separatly without using union/union all it ran sucessfully..

thanks in advance..