SELECT
    SUM(Total_P + Total_A) AS Total
FROM
    (SELECT
        Pi.IdItem,
            SUM(Pi.Quantidade * Pi.ValorUnit) AS Total_P,
            0 AS Total_A
    FROM
        pedidoitemaux Pi
    GROUP BY Pi.IdItem UNION ALL SELECT
        Pa.IdItem,
            0 AS Total_P,
            SUM(Pa.Quantidadead * Pa.Valorad) AS TotalA
    FROM
        pedidoitemadaux Pa
    GROUP BY Pa.IdItem)