Reference between XML files

I want to refer between different XML files. For example if I have a schema for patients with their properties and one with diseases and their properties in a database in different collections. I create an ID for the diseases and in a single xml file with one patient I want to refer to a disease ID from the Database. If I use therefor an IDREF XML Spy cannot validate this file because the appendant ID doesn’t exist in this xml file. Can anybody give me some ideas how to solve these problem?

Could you please post your schema files to this forum.