Hello Phillip! Great question! Fleet does not support connecting to multiple standalone redis servers. The configuration options for Redis only allow specifying a single Redis server address. Redis cluster is supported however if you were interested in scaling horizontally.
I'm adding a link to our configuration documentation for reference.
https://fleetdm.com/docs/configuration/fleet-server-configuration#redis
Out of curiosity, if you were able to accomplish the setup you were looking for, how would you use it or how would that benefit your use of Fleet?