stub-addr failover

Using unbound 1.4.22. I have a config like so:

stub-zone:
name: “example.com
stub-addr: 192.168.1.1
stub-addr: 192.168.1.2

If 192.168.1.1 is not responding, will queries automatically go to 192.168.1.2? If so, what kind of event is required for 192.168.1.1 to be determined as unreachable?