Concatenating XML fragments

I won't try to guess why you're doing this in separate SELECT statements, or why you would want to recombine using SQL instead of PL/SQL. But, here is a working example to do what I think you are trying to do.

DECLARE
  XMLSTRING1    xmlType;
  XMLSTRING2    xmlType;
  XMLSTRING3    xmlType;
BEGIN
  
  SELECT xmlElement("FIRST_STRING", xmlForest(ename "COL1", job "COL2"))
    INTO xmlString1
    FROM emp
    WHERE ROWNUM = 1;

  DBMS_OUTPUT.put_line( xmlString1.getStringVal() );
      
  SELECT xmlElement("SECOND_STRING", xmlForest(sal "COL1", mgr "COL2"))
    INTO xmlString2
    FROM emp
    WHERE ROWNUM = 1;

  DBMS_OUTPUT.put_line( xmlString2.getStringVal() );
     
  SELECT xmlElement( "THIRD_STRING", xmlString1, xmlString2 )
    INTO xmlString3
    FROM DUAL;    

  DBMS_OUTPUT.put_line( xmlString3.getStringVal() );

END;

The first and second SELECT staements gather two XML fragments and the third statement uses XMLElement to combine them. I used DBMS_OUTPUT to display the intermediate results.

Hope that helps.