My general recommendation is to have licensing questions discussed between the end customer and their account manager at OSIsoft.
The reasoning you describe indicates failover purpose but I am confused by the last sentence that talks about one AF Server for the MDB content and another one as pure "AF Server".
To my knowledge, if the PI System license includes HA, failover purpose for PI AF Server is as well included. This is something to confirm with the account manager.
Technically PI AF Server can be installed redundantly (please see deployment scenarios in PI AF Server 2012 Installation Guide). Inside PI AF Server one can create different "Databases" for different purposes. My recommendation is to migrate MDB content into a separate AF Database.
Thanks for your answer. I will ask the customer to check with their account manager.
This does not concern failover. It is just that the PI-server sits behind a firewall together with the PI AF needed for the MDB. The PI/AF that is visible for the client-applications needs to live on the other side of the firewall accessing the PI-server on port 5450 through the firewall.
The PI AF server will also be maintained by other people than the PI-server.