Multiple Third Party Designees are setup through multiple Users. To setup multiple Users, click the User button on the 1099-etc Main Menu, setup a new 3-character User Code and enter the information for each Third Party Designee. Then selecting the appropriate User code will print the associated Third Party Designee information.
In AMS Payroll, you can enter a default User Code for each payer on the Payer>Payer Edit>Miscellaneous screen. This will always use the Third Party Designee information that is setup for this user.