Actually, it is easy.
First, Create a draft Invoice:
Go to 'New Invoice' window.
Select any customer, add desired items to the invoice, and review the details and total.
Click 'Save as Draft' from the Record pull-down-list at the bottom of 'New invoice' window.
Once the draft invoice is created, it can be used to create as many invoices as needed with different invoice numbers.
Go to 'New Invoice' window.
From the 'Create From' pull-down-list on top-right, select 'Draft Invoice'.
It should show you the draft invoice you created in the first step. Select the draft invoice.
The 'New Invoice' window should now be filled with the same details as the selected draft invoice. Now simply change the customer.
Review the details and total, and click 'Record Only' to create a new invoice.