For the next time I need this because my editor is buggy:
<quer:projection alias="Ident" xmlns:quer="http://www.taleo.com/ws/integration/query"> <quer:concatenate> <quer:field path="Candidate,Number"/> <quer:string>_</quer:string> <quer:field path="Requisition,Number"/> </quer:concatenate> </quer:projection>