try this..

=IF(ISERROR(INDEX([Backlog 10-5.xls]Old Backlog'!$A$2:$U$20000,MATCH(A2&B2,[Backlog 10-5.xls]Old Backlog'!$A$2:$U$20000&ROW($A$2:$A$20000),0),9)),"",INDEX([Backlog 10-5.xls]Old Backlog'!$A$2:$U$20000,MATCH(A2&B2,[Backlog 10-5.xls]Old Backlog'!$A$2:$U$20000&ROW($A$2:$A$20000),0),9))

Confirm with Ctrl+Shift+Enter and not just Enter