Key and Keyref in different files

Hi there,
 
I am more ore less new to the world of XML and I have a big problem. I
try to find a mapping form the object oriented world to the hierarchical
world of XML. If u only consider simple objects the mapping is very
simple, e.g. a person with a first and a lastname and another object
publication with a title and an abstract. This will lead to 2 small and
very simple XSD and XML files. The problem occurs when I try find a
mapping for relations between the 2 objects, e.g. a person has written a
publication and a publication has an author. For this mapping I need
something like objectreferences transfered to world of XML. The only
possiblities I found so far to build references between elements in XML
are ID/IDREF and KEY/KEYREF BUT this only works (I think) if all data
exists in one single file! And that is not what I want, I need to split
up the data in several files. Does anyone have any suggestions how to
build those references between several files? I would be very happy
about any help.
 
Greetz,
Nick