stub-addr failover

[Christopher Hearn via Unbound-users]

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?