I want to merge two (or more) sales order together so I can sent one invoice to the customer.

When I select two sales orders and try to merge them together (this function is under the button more, in the list view). And select group invoices. I got an warning message: 

Warning!
You shouldn't manually invoice the following sale order SO00407.

pressing the OK button bring me back to the Make invoices popup window.

How can I merge two sales orders correctly, or is there a possibility to merge to invoices to one?