I believe the scenario you're looking at requires a SignalR Backplane to be setup.
Here's a relevant selection from the article but you'll want to read the full thing to answer your specific questions:
Each server instance connects to the backplane through the bus. When a message is sent, it goes to the backplane, and the backplane sends it to every server. When a server gets a message from the backplane, it puts the message in its local cache. The server then delivers messages to clients from its local cache.