Joining two XML-files

I have 2 XML files obtained from an old legacy system.
(each one is a copy of a relational table).

Here is an excerpt of every one
file1.xml
<data>
    <row att1='foo' att2='morefoo" ... />
    <row att1='foo2' att2='morefoo2" ... />
...
</data>

file2.xml
<data>
    <row fld1='foo' fld2='otherfoo" ... />
    <row fld1='foo2' fld2='otherfoo2" ... />
    <row fld1='foo2' fld2='anotherfoo2" ... />
...
</data>

These files are related through the commom value of an attribute.
In the previous example file1.att1  is found in file2.fld1 
I need to produce a third file where get mixed some attributes
of the related "records".For example:

file3.xml
    <row att1='foo' att2='morefoo" fld2='otherfoo" />
    <row att1='foo2' att2='morefoo2" fld2='otherfoo2" />
    <row att1='foo2' att2='morefoo2" fld2='anotherfoo2" />
...
</data>

I'm wondering what is the "best practice" to achieve this.
Any help will be appreciated.

TIA
Jorge


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