To add one invoice number to two purchaser orders as a supplier follow the steps below:
Log in to your supplier portal.
Click on the Orders tab.
Click on Batch Orders.
Select the Purchaser from the drop down options.
Select the Site from the drop down option.
Enter the Purchaser Orders into the Order ID box, add one order per line.
You will then be able to add the invoice number to all the orders created under the Batch.