See the question and my original answer on StackOverflow

You can use the idl import directive. The Windows SDK .idl files is full of these (like import "unknwn.idl";). So, in processor.idl:

import "face.idl";  

[
    object,
    uuid(6E0537CC-232A-4E73-A625-358591CA9231),
    pointer_default(unique)
]
interface IProcessor
{
    HRESULT ProcessFace(IFace *pFace);
};

Of course, this will need the .idl files (and generated .h and various artifacts, if any, for compilation phase) to be present in accessible directories.