Terminology gets mixed up, so lets call it server 1, server 2
To start: server 1 = AMP, server 2 = failover
In your proposed scenario, server 1 = down, server 2 = AMP after failover
The next part depends on preference, but ideally, most customers would bring server 1 back online, and then restore it to being the active AMP by taking a backup from server 2 and restoring. After that, they'd use the /var/airwave-backup/watcher backup to restore on server 2 to return the server to failover.
But it's perfectly okay to change and let server 2 remain AMP while bringing up server 1 as failover. That is a good option as well in which case you can take the watcher backup from server 2 to restore on server 1, or go through the setup as you would since it doesn't take that much effor to setup a failover server.