ah, sorry, I see my mistake, the @ comes after -
SELECT requestID, orderID
  FROM
  (SELECT baserequestID, requestID, orderID, ROW_NUMBER() OVER (PARTITION BY baserequestID ORDER BY requestID DESC, orderID DESC) AS recentness
     FROM
     (SELECT CASE WHEN CHARINDEX('-', requestID) = 0 THEN requestID ELSE LEFT(requestID, LEN(requestID) - 2) END AS baserequestID, requestID, orderID
        FROM ids
      UNION ALL
      SELECT CASE WHEN CHARINDEX('-', requestID) = 0 THEN requestID ELSE LEFT(requestID, LEN(requestID) - 2) END AS baserequestID, requestID, orderID
        FROM ids_done) u
     ) rn
  WHERE rn.recentness = 1