Really ugly recursive CTE solution (I'm hoping someone comes up with a better solution). The performance for this will scale terribly.

WITH CTE AS (

SELECT ID, TranID,

CASE WHEN a.N = 1 THEN OriginCode ELSE DestinationCode END AS code,

ROW_NUMBER() OVER(PARTITION BY TranID ORDER BY ID, a.N) AS pos

FROM #Test

CROSS JOIN (SELECT 1 UNION ALL SELECT 2)a(N)),

CTE2 AS (

SELECT ID, TranID, code, CAST(code AS VARCHAR(MAX)) AS overallCode, pos

FROM CTE

WHERE pos = 1

UNION ALL

SELECT b.ID, b.TranID, b.code, overallCode + b.code, b.pos

FROM CTE2 a

INNER JOIN CTE b ON a.TranID = b.TranID AND a.pos < b.pos AND a.code <> b.code)

SELECT TranID, overallCode

FROM (SELECT TranID, overallCode, ROW_NUMBER() OVER(PARTITION BY TranID ORDER BY LEN(overallCode) DESC)

FROM CTE2) a(TranID, overallCode, ItemNumber)

WHERE ItemNumber = 1;

Results: -

TranID overallCode

----------- --------------------

201205 AMDDOHNBOADDDELAMD

201206 IXJRMI