Hi there,
We are missing some information here, like what is the method used for sending the invoice.
But if you are using default methods (or business document management) you could set up a specific rule in print management.
So go to print management (account receivable --> setup --> forms --> form setup --> print management). Select the document you want to use (sales order = customer invoice), right click it and select new. On the first node you have to set whether it regards regular prints or copies. Select the proper node and create a second node under it by right clicking and selecting new again.
In this node you can add filters, set a specific filter for invoice account HQ and customer account Subsidiary (in the same filter, both fields are in customer invoice journal). Then select how you want to send the document, I'll asume email here. Enter the email address of both the subsidiary and HQ manually.
Let me know if this worked for you or what your specific requirements are.