Not 100% sure I understand what you are trying to do, but I'm assuming your orders always have either one line or two.

Not one of the cleverest solutions I've ever constructed, but here it is.

WITH OneRowOrders AS

(

SELECT OrderID, ProjectID, ProductID, ProjectAmt, ID=MIN(ID)

FROM #Orders

GROUP BY OrderID, ProjectID, ProductID, ProjectAmt

HAVING COUNT(*) = 1

)

SELECT ID, OrderID, ProjectID, ProductID, ProjectAmt

,ProductAmt = ISNULL(ProductAmt +

LEAD(ProductAmt, 1) OVER

(

PARTITION BY OrderID, ProjectID, ProductID, ProjectAmt

ORDER BY ID, rn

), 0)

FROM

(

SELECT ID, OrderID, ProjectID, ProductID, ProjectAmt, ProductAmt

,rn=ROW_NUMBER() OVER (PARTITION BY OrderID, ProjectID, ProductID, ProjectAmt ORDER BY ID)

FROM #Orders

UNION ALL

SELECT ID, OrderID, ProjectID, ProductID, ProjectAmt, 0, 2

FROM OneRowOrders

) a

ORDER BY ID, rn;

It does assume that if you are posting in a SQL 2012 forum you are running on SQL 2012.

Edit: Fixed indentation.