As we are scaling up, we've added a front end gateway and added redundancy to our on prem gateways.
For the gateway network: Should I establish the connection from the front end to both the primary and the backup, or is only one needed?
I've been informed originating the connection from the redundant primary conveniently also sources one form the backup to the remote(in this case our front end).
You will need to connect your Front End to both your Primary and Backup Gateway. This way, information that is passed to the Backup will be accessible to your Front End Gateway. For example, if you use your Primary Gateway as a remote tag provider and failover to your Backup, you will be unable to see new values for your tags in that tag provider unless your Front end is also connected to your Backup.
More on setting up redundancy in Ignition here: