This would work but I am wondering if there could be something simpler:
SELECT Client.Numero
FROM Client
LEFT JOIN Invoice ON Invoice.Numero=Client.NoInvoice AND Invoice.SecondCondition=True
LEFT JOIN Inventory ON Inventory.Numero=Client.NoInventory AND Inventory.SecondCondition=True
WHERE (EXISTS (SELECT Invoice.Numero FROM Invoice WHERE Invoice.Numero=Client.NoInvoice AND Invoice.SecondCondition=True) OR
EXISTS (SELECT Inventory.Numero FROM Inventory WHERE Inventory.Numero=Client.NoInventory AND Inventory.SecondCondition=True))
