Unique constrain using XML Schema

[Anoop Krishnan]

I have an xml file as given below.

 
<ng:DataModel>
       <ng:FidSource provides="EnumeratedData"   
                     id="FSDerivativeUnderlyingType">
<ng:Fid code="UL_TYPE" id="EnumValue" /></ng:FidSource>
<ng:FidSource provides="EnumeratedData" id="FSCertificateType">
<ng:Fid code="CER_TYPE" id="EnumValue" /></ng:Fid>
</ng:FidSource>
</ng:DataModel>

I want FidSource@id/Fid@id combination unique
How can it be achieved in an xml schema definition using <xs:uinique> constrain
I tried the folowing but it is not valid

 
<xs:unique name="uniqueFidsInFidSources">
  <xs:selector xpath="ng:FidSource/ng:Fid" />
  <xs:field xpath="@id" />
  <xs:field xpath="../@id" />
</xs:unique>