Does it only let the devices use the primary DNS until failure then use the secondary DNS?
or does the devices use both at the same time all the time?
For context, I have a single pi-hole set up and I want the devices to only use the pi-hole unless it failed then use public dns such as 8.8.8.8 or 1.1.1.1.
Most if not all devices (looking at you Windows) do not use the secondary DNS at all, not even at failover, they take their due time to use the secondary one. Best practice would be to use a VIP as primary DNS that will load balance the requests to two DNS.