Prueba de esta forma:
Código SQL:
Ver originalSELECT Tasa, SUM(Importe)
FROM (
SELECT DISTINCT [Tasa1] AS Tasa, SUM(Neto1+Neto2) AS Importe FROM Compras
WHERE (Tasa1 IS NOT NULL)
GROUP BY Tasa1
UNION
SELECT DISTINCT [Tasa2] AS Tasa, SUM(Neto1+Neto2) AS Importe FROM Compras
WHERE (Tasa2 IS NOT NULL)
GROUP BY Tasa2)
GROUP BY Tasa