Yes, your two ZDs can each maintain local AP connections if the WAN link between them goes down.
When you configure the two ZDs as Smart Redundancy peers on Configure/System, also go to
Configure/Access Points, and under AP Policies configure both ZoneDirector IP addresses under
the Limited ZD Discovery section. You can specify which you would prefer to be primary ZD here.
Your DHCP server(s) may be configured to provide IP addresses to your APs, and they should
provide both ZD IPs in Option 43, and/or in DNS for 'zonedirector.localdomain' hostname.
If the APs cannot reach the primary ZD, perhaps for temporary WAN outage, they will connect to
the ZD that is reachable, and continue to operate and provide local services. You could end up
with two ZDs and half of your APs connected to each of them.