Hi,
You could do something very similar from an architecture point of view by simply having 1 mysql database on the master machine. You would need to make sure to configure the connection in Ignition in such a way that it would work from the backup too (the configuration is sync’d between the machines). Then, configure your “history mode” on the backup as “partial”.
When the master goes down, the backup will take over. However, in partial mode, all of the data will just be cached. When the master comes back up, the backup will see that it has been running longer, and will forward all of the data to the database.
Hope this helps, let me know if something isn’t clear.