Which element for cross-document xref?

Hi,

I'm using xref happily in my document. Now I like to split the
source into chapters, xincluded by a master file. That way any
xref from one file to the another becomes invalid, because an
IDREF (xref linkend) has to point to an ID in the same document,
right? Shall I use another element instead of xref?

Thanks in advance!

Cheers, WB