In the Exchange Org, if the "failover" exchange server is already known, meaning that if you look in the Exchange Org and see the other server and mailstores, theoretically, you should be able to the run the Unity Messaging store wizard and add that server into the Unity. Unity will be able to deliver to that mailstore then.
However, it if it not mention in the installation guide, it may not be TAC supported. You can always open a ticket up with TAC to check with them.
Looking at the link, what I think is happening is this:
ExMbx1 has it's own mailstores, then a "standby " mailstore of ExMbx2. When ExMbx2 goes down, Exchange will automatically update the users mailstores in the system from ExMbx2 to ExMbx1.
John Doe is on ExMbx2. This server goes down. ExMbx1 notices ExMbx2 is down and essentially activates and creates changes in it's own database that ExMbx2. All users associated to this server are now associated to ExMbx1.
DNS must take care of ExMbx2 record and there is a secondary record for ExMbx2 which is ExMbx1.
John Doe on the client side does not know that this server is down. His mail has been replicating to that standby databases on ExMbx1.
If you ever setup Outlook and have multiple Exchange servers, you will have noticed how Exchange will update the server name for you when you are setting up. For example, you are setting up John Doe, you enter in Mail01 for the server and then hit check name, it comes back and finds him on Mail03 and updates the client accordingly.
My guess is when you run the Mailstore wizard for Unity, add in the standby databases that will most likely show up after this failover setup is installed.
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
If you want to download them you have to prefix "file get " and you can add one of the options (re...