Validation of XML Schema (XSD) element groups

I have searched the mailing list archives (user and dev) and checked
Jira but I cannot find anything that exactly matches this potential
problem (most if not all of the issues are with the combination of
"group" with "choice"). This may mean that I am misunderstanding the XSD
spec. but I am sure that someone will let me know if that is the case!

I am trying to create a schema definition for an element that must
contain either or both of two sub-elements. I do not want to make both
sub-elements optional because I want to ensure that users send at least
one of them but I cannot use "choice" because I want users to be able to
send both elements if necessary. I thought that element groups might be
the answer and so this is the schema I created (test_group.xsd):

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema";>

  <xsd:element name="root">
    <xsd:complexType>
      <xsd:sequence>
        <xsd:element ref="elements" minOccurs="1"
maxOccurs="unbounded"/>
      </xsd:sequence>
    </xsd:complexType>
  </xsd:element>

  <xsd:element name="elements">
    <xsd:complexType>
      <xsd:sequence>
        <xsd:element name="element1" type="xsd:string" minOccurs="1"
maxOccurs="1"/>
        <xsd:group   ref="element_group"               minOccurs="1"
maxOccurs="1"/>
      </xsd:sequence>
    </xsd:complexType>
  </xsd:element>

  <xsd:group name="element_group">
    <xsd:sequence>
      <xsd:element name="element2" type="xsd:string" minOccurs="0"
maxOccurs="1"/>
      <xsd:element name="element3" type="xsd:string" minOccurs="0"
maxOccurs="1"/>
    </xsd:sequence>
  </xsd:group>

</xsd:schema>

Note that both elements within the group are optional but the group is
specified as mandatory within the definition of "elements" (minOccurs =
1). This is the test data:

<?xml version="1.0"?>
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
      xsi:noNamespaceSchemaLocation="test_group.xsd">
  <elements>
    <element1>aaaa</element1>
  </elements>
  <elements>
    <element1>aaaa</element1>
    <element2>bbbb</element2>
  </elements>
  <elements>
    <element1>aaaa</element1>
    <element3>cccc</element3>
  </elements>
  <elements>
    <element1>aaaa</element1>
    <element2>bbbb</element2>
    <element3>cccc</element3>
  </elements>
</root>

I expected the first occurrence of "<elements>" to fail (neither
<element2> nor <element3> is present) but the file passed validation. Is
it Xerces-J (2.8.0), the schema or my understanding that is incorrect
and, if it is not the Xerces-J, is there a way to do what I want in a
W3C schema?

Note: Validation against a DTD showed the same problem.

Thanks,
Andrew

All opinions are completely my own - after all, who else would want to
take credit for them?