When restoring a backup intended to work as a failover system, be sure to restore the DiRT backup to a stand alone server first, make sure it's running properly then run the failover configuration wizard on that server.  You cannot restore a DiRT backup onto either a primary or secondary failover pair, it will not work properly.

If you have two Unity servers configured for fail over it’s best to backup the Unity server configured as the fail over server, not the primary if possible.  The primary Unity server will have additional links in SQL setup for replicating to the fail over server database and these links will be broken when you restore to a new server.  If you then try to reconfigure fail over capabilities on that server it will fail.  There’s no way to automatically remove these flags from the backed up database so if you have backed up the primary Unity server you will need to manually clean up the SQL properties after restore.  The DiRT restore notices the fact that the restored database has these links and will pop up a warning dialog at the end of the restore reminding you of these (probably why you’re here reading this now).  The steps to clean up the SQL replication links are as follows:

1. Open SQL Enterprise Manager.
2. Expand Microsoft SQL Servers -> SQL Server Group -> "Server Name"
3. Right-click on the Replication folder and select Configure Publishing, Subscribers, and Distribution...
4. A wizard will pop up.
5. Click Next>.
6. Click Next>.
7. Click Next>.
8. Click Yes.
9. Click Next>.
10. Click Finish.
11. Click OK.
12. Click Close.  
13. Right click on the Replication folder and select Disable Publishing…
14. A wizard will pop up.
15. Click Next>.
16. Choose the option labeled Yes, disable publishing on "Server Name".
17. Click Next>.
18. Click Next>.
19. Click Finish.
20. An error will pop up.
21. Click OK to close the error.
22. Click Cancel.

At this point it should now be possible to reconfigure the restored Unity server for fail over again.