See references:

http://www.xfront.com/Embedding-Various-Components-into-an-XML-Component.docx

Also this fragment:

here's probably another technique (using XML Schema 1.1 assertions and complexType restriction) to achieve the stated instance validation
objectives:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://www.example.org/header"
xmlns:header="http://www.example.org/header"
elementFormDefault="qualified">

<xs:element name="Header">
<xs:complexType>
<xs:complexContent>
<xs:restriction base="header:TYPE_BASE">
<xs:sequence>
<xs:element name="Author" type="xs:string" />
<xs:element name="Report"
targetNamespace="http://www.example.org/report"
xmlns:report="http://www.example.org/report">
<xs:complexType>
<xs:sequence>
<xs:any processContents="lax"/>
</xs:sequence>
<xs:assert test="report:Title"/>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
</xs:element>

<xs:complexType name="TYPE_BASE">
<xs:sequence>
<xs:any processContents="lax" maxOccurs="2" />
</xs:sequence>
</xs:complexType>

</xs:schema>