Hi Ron ...
Yes It Is Possible you can create a Object and sum Try this
Select Sum(tt.QTY) from (
SELECT
SUM(T0.[Quantity]) as QTY FROM INV1 T0
INNER JOIN DLN1 T1 ON T0.BaseEntry = T1.DocEntry and T0.BaseLine = T1.LineNum
INNER JOIN RDR1 T2 ON T1.BaseEntry = T2.DocEntry and T1.BaseLine = T2.LineNum
WHERE
T2.[DocEntry] = 19 and T0.BaseType = 15 and T1.BaseType = 17
UNION all
SELECT
SUM(T0.[Quantity]) as QTY FROM INV1 T0
INNER JOIN RDR1 T2 ON T0.BaseEntry = T2.DocEntry and T0.BaseLine = T2.LineNum
WHERE T2.[DocEntry] = 19 and T0.BaseType = 17 )
as TT
Hope Helpful
Regards
Kennedy