AFAIK there is no general solution.  You certainly could do something with
user defined functions but that is quite inefficient unless your database
model allows to define indexes on these function columns.

You better choose a different storage scheme for your files, use an XML
enabled database or use proprietary code to figure these things out.  If
you just need to do it once, then I'd probably write a script that does
the job.  Dunno however what else will be part of your analysis.

Kind regards

   robert

Followings might not the way you want. But, it worked on DB2 UDB.
CREATE TABLE File_XML
(file        CHAR(8)     NOT NULL
,line        SMALLINT    NOT NULL
,desc        VARCHAR(30) NOT NULL
) IN USERSPACE1
;

INSERT INTO File_XML
VALUES
('file1', 1, '<mytag id="aaa"/>')
,('file1', 2, '<mytag id="bbb"/>')
,('file1', 3, '<mytag id="ccc"/>')
,('file2', 1, '<mytag2 lookup="aaa"/>')
,('file2', 2, '<mytag2 lookup="aaa"/>')
,('file2', 3, '<mytag2 lookup="bbb"/>')
;

SELECT f1.file
    , f1.line
    , f1.desc
    , f2.file
    , f2.line
    , f2.desc
 FROM File_XML f1
    , File_XML f2
WHERE f1.file = 'file1'
  AND f2.file = 'file2'
  AND SUBSTR(f1.desc, LOCATE('id="', f1.desc)+4,
             LOCATE('"/>', SUBSTR(f1.desc,
                                  LOCATE('id="', f1.desc)+4)) -1)
    = SUBSTR(f2.desc, LOCATE('lookup="', f2.desc)+8,
             LOCATE('"/>', SUBSTR(f2.desc,
                                  LOCATE('lookup="', f2.desc)+8)) -1)
;
------------------------------------------------------------------------------

FILE     LINE   DESC                           FILE     LINE   DESC
-------- ------ ------------------------------ -------- ------
------------------------------
file1         1 <mytag id="aaa"/>              file2         1 <mytag2
lookup="aaa"/>
file1         1 <mytag id="aaa"/>              file2         2 <mytag2
lookup="aaa"/>
file1         2 <mytag id="bbb"/>              file2         3 <mytag2
lookup="bbb"/>