Activating second invoice numbering

To activate a second set of invoices navigate to Fiscal Management → Tax Settings and click edit icon on the company row.
From the next screen scroll down to the invoices section and value the checkbox "enable second set of invoices".
Below fill in the two fields highlighted in red:

• Next invoice (2nd series): enter the number from which you intend to start the second progressive numbering of invoices (or credit notes)
• Invoice number format (2nd series): enter the format of the invoice number (it will be sufficient to copy and paste that of the first invoice series)
  N.B. {n} and {y} represent two placeholders for invoice number and year. {n} will incrementally increase as invoices are issued and {y} will incrementally increase as years pass. Therefore, they will not need to be changed unless you want to change the format of the invoice number.
  In case you intend to use the second numbering for issuing credit notes, you will be able to modify the numbering format by inserting additional letters to further distinguish credit notes from invoices.
  

Should you opt to do as from the image above, what is written outside the curly brackets will be fixed on all numbers of invoices/credit notes issued; the characters inside the curly brackets, on the other hand, as explained just above, are merely placeholders that will incrementally increase in an automated way.

Click the button at the bottom of the page "Apply Changes" to finish the operation.