It's possible to exclude records when loading the data. You could not load the French customers in the Customers table:
Load * from Customers.qvd (qvd) where Country <> 'France';
// load only the customers where CustomerID exists in the table Customers
Load * from Transactions.qvd(qvd) where exists(CustomerID);
If you want to load all customers into the Customers table you could drop the Customers table after load the Transactions table and then create the Customers table anew by load all the data from the Customers qvd.
If you need the French customers and their transactions data on other tabs then you need to load the complete qvd files.
It is possible to excluse customers, let's suppose that both qvd's have customer_id field so qvd are linked by this field.
Load * from Customers.qvd;
Load * from Transactions.qvd;
Load * resident AllData where Customer_Country = 'France';
Drop table AllData;
Let me know ...