The best practice is to create a separate Contact Record for the CPA then link the Contacts by marking them as Associates. Though the CPA can be added as an Informational Contact, this limits reporting possibilities.
After both Contacts are created, add the association within the Associates Tab > Associates of either Record. The Relationship link will appear on both Records regardless of which it was created from.
This allows the ability to report on the association via the Global Associate Report. Informational Contacts are not included in this report. This also allows the flexibility of adding the CPA to another client's Record without needing to input all the information again as with an Informational Contact.