DECLARE @tblData TABLE(TransferID int, SystemID int, ToSystemID int, TransferDate date)
insert into @tblData 
SELECT 1, 15, 30, '09/09/2011' union all
SELECT 2, 8, 2, '10/01/2008'


DECLARE @tblMain TABLE(CustID int, IncDate date,SystemID int)
insert into @tblMain 
SELECT 11, '15/02/2007' ,15 union all
SELECT 11, '18/01/2010', 30 union all
SELECT 7, '26/11/2000', 2 union all
SELECT 3, '12/12/2000', 2 union all
SELECT 3, '18/03/1999', 8

--SELECT CustID, d.SystemID, m.SystemID, d.ToSystemID,IncDate, TransferDate
UPDATE m 
SET IncDate = TransferDate
FROM @tblData d JOIN @tblMain m ON d.ToSystemID = m.SystemID
WHERE CustID IN (SELECT CustID FROM @tblMain GROUP BY CustID HAVING COUNT(*) = 2)

SELECT * FROM @tblMain