Hello,
Assuming you have parent schema (parent.xsd) that refers to child schema (child.xsd).
Inside of the parent schema (parent.xsd) you have include statement:
"<xs:include schemaLocation="child.xsd"/>".
Assume, you have created two sap pi external definition objects XI_parent and XI_child that refers to aforementioned schemas.
All you need to do is in XI_child object in field "Source" put "child.xsd". That will make XI_parent object aware of "child.xsd" represented by XI_child (External References tab).
Regards,
Jakub Turminski