Select an appointment you would like to invoice and choose Complete & Invoice.
The insurance record created in the patient's profile set as the default will be selected.
If applicable and necessary, adjust the Third-Party to read as the primary payor.
When ready to proceed, select 
.
.
The Receive Payment window will display.
Select from the Paid By menu, the option Third-Party Coverage.
The Third Party will have the other insurance selected. Adjust this if necessary.
Select Apply Payment when ready to assign the remaining amount owed to the secondary insurance.
This will create a second invoice for this secondary insurer.