You can use the Export Sales Journal function under File/Export. When you export use the invoice number filter by the 1st invoice number and Filter TYPE is equal to Sale, and under the options tab choose include headings, it is saved to Documents Sales, or you can save to any location you want.  Export the 2nd invoice in the exact same manner however under the Option Tab, do not choose to include headings and choose to append without asking. This will add each export to the same Sale document as a CSV file. Export all of your invoices this way. using the invoice number filter.

You would have to make a few changes to the sale.csv before you could import it as 1 invoice. 

If this is something you want to try, I can be more specific. PM me for more details.

You can use the Export Sales Journal function under File/Export. When you export use the invoice number filter by the 1st invoice number and Filter TYPE is equal to Sale, and under the options tab choose include headings, it is saved to Documents Sales, or you can save to any location you want.  Export the 2nd invoice in the exact same manner however under the Option Tab, do not choose to include headings and choose to append without asking. This will add each export to the same Sale document as a CSV file. Export all of your invoices this way. using the invoice number filter.

You would have to make a few changes to the sale.csv before you could import it as 1 invoice. 

If this is something you want to try, I can be more specific. PM me for more details.

Thank you.  I will PM for more details.