Assuming you’re indeed referring to a load balancer scenario, this is actually a very simple variation of the Service Discovery pattern.
Every instance of you Hangfire server that starts up, registers an ID unique to that server instance (e.g machine name) in a Db table / Redis dictionary.
When you add a job to the queue that you want to run on all registered servers, you first query whatever store you used for your unique server IDs, get all registered servers and add the same job for each server with a unique ID for each job. That should give you the desired effect. They won’t fire at exactly the same time but that’s the nature of how queues work.
Then when scaling down your servers, either automatically or manually, you simply make sure to deregister the server ID from wherever you stored it as part of the shutdown task.