I have created a schema, but I got an error validating it. Is it possible to 
have 2 ds:Reference elements (with different attributes)?

Error message:

Element "ds:Reference" occurs more than once in the same content model but 
does not refer to top-level type definition.

Schema:

<schema xmlns="http://www.w3.org/2001/XMLSchema" 
xmlns:ds="http://www.w3.org/2000/09/xmldsig#" 
targetNamespace="http://www.w3.org/2000/09/xmldsig#" 
elementFormDefault="qualified" version="0.1">
<!-- Basic Types Defined for Signatures -->
<simpleType name="CryptoBinary">
<restriction base="base64Binary"/>
</simpleType>
<!-- Start Signature -->
<element name="Signature" type="ds:SignatureType"/>
<complexType name="SignatureType">
<sequence>
<element ref="ds:SignedInfo"/>
<element ref="ds:SignatureValue"/>
<element ref="ds:KeyInfo"/>
</sequence>
<attribute name="Id" type="string" use="required" fixed="sig1"/>
</complexType>
<element name="SignatureValue" type="ds:SignatureValueType"/>
<complexType name="SignatureValueType">
<simpleContent>
<extension base="base64Binary">
<attribute name="Id" type="ID" use="optional"/>
</extension>
</simpleContent>
</complexType>
<!-- Start SignedInfo -->
<element name="SignedInfo" type="ds:SignedInfoType"/>
<complexType name="SignedInfoType">
<sequence>
<element ref="ds:CanonicalizationMethod"/>
<element ref="ds:SignatureMethod"/>
<element name="Reference">
<complexType>
<sequence>
<element ref="ds:DigestMethod"/>
<element ref="ds:DigestValue"/>
</sequence>
<attribute name="URI" type="anyURI" use="required" fixed="#data1"/>
</complexType>
</element>
<element name="Reference">
<complexType>
<sequence>
<element ref="ds:DigestMethod"/>
<element ref="ds:DigestValue"/>
</sequence>
<attribute name="URI" type="anyURI" use="required" fixed="#tra1"/>
</complexType>
</element>
</sequence>
<attribute name="Id" type="ID" use="optional"/>
</complexType>
<element name="CanonicalizationMethod" 
type="ds:CanonicalizationMethodType"/>
<complexType name="CanonicalizationMethodType" mixed="true">
<attribute name="Algorithm" type="anyURI" use="required"/>
</complexType>
<element name="SignatureMethod" type="ds:SignatureMethodType"/>
<complexType name="SignatureMethodType" mixed="true">
<attribute name="Algorithm" type="anyURI" use="required"/>
</complexType>
<element name="DigestMethod" type="ds:DigestMethodType"/>
<complexType name="DigestMethodType" mixed="true">
<attribute name="Algorithm" type="anyURI" use="required"/>
</complexType>
<element name="DigestValue" type="ds:DigestValueType"/>
<simpleType name="DigestValueType">
<restriction base="base64Binary"/>
</simpleType>
<!-- End SignedInfo -->
<!-- Start KeyInfo -->
<element name="KeyInfo" type="ds:KeyInfoType"/>
<complexType name="KeyInfoType" mixed="true">
<choice>
<element ref="ds:X509Data"/>
</choice>
<attribute name="Id" type="ID" use="optional"/>
</complexType>
<!-- Start X509Data -->
<element name="X509Data" type="ds:X509DataType"/>
<complexType name="X509DataType">
<sequence>
<choice>
<element name="X509Certificate" type="base64Binary"/>
</choice>
</sequence>
</complexType>
<!-- End X509Data -->
<!-- End KeyInfo -->
<!-- End Signature -->
</schema>