The only way you can include a second invoice/purchase order would be to use a second field dedicated to this. You can rename one of your custom fields or any other field you are not using to be your second purchase order/invoice field.

To rename the field go to the Customize Menu at the top of the screen and select Customize Fields

Find the field you wish to rename and change the title on the right side to the new name.

As a note, when you are doing your import the field will still show as the old name but once you are in the company the new name will show again.