SQL> create table t ( x xmltype );

Table created.

SQL>
SQL> begin
  2    insert into t values (
  3      xmltype.createxml('<departments>
  4          <dept>
  5            <deptno>123</deptno>
  6            <dname>My Department</dname>
  7          </dept>
  8        </departments>'));
  9
 10    insert into t values (
 11      xmltype.CREATEXML(
 12       '<emp>
 13          <empno>456</empno>
 14          <ename>John Smith</ename>
 15        </emp>'));
 16  end;
 17  /

PL/SQL procedure successfully completed.

SQL>
SQL> select xmlelement("MERGED", XMLAGG(x)) from t;

XMLELEMENT("MERGED",XMLAGG(X))
-----------------------------------------------------------
<MERGED><departments>
  <dept>
    <deptno>123</deptno>
    <dname>My Department</dname>
  </dept>
</departments><emp>
  <empno>456</empno>
  <ename>John Smith</ename>
</emp>
</MERGED>