The network configuration for the X0 and X1 adapters is entered directly into the underlying Windows Network Settings and it uses standard Windows behavior with regards to DNS servers and lookups.
Safeguard sends the name query to the 1st DNS server on the preferred adapter's list (X1) and waits (1 second) for a response. If Safeguard does not receive a response from the 1st DNS server within (1 second), it will then send the name query to all DNS servers on the preferred adapter's list (X1) and wait (2 seconds) and so on... until (8 seconds) in which case if Safeguard has not received a response from any server, it will respond with a timeout.
The Safeguards DNS client service internally will keep track of which servers answer the name query quickly and it will move the servers up or down on the list based on how quickly they reply to name queries.
The DNS query will stop when a response has been received, regardless of whether it was successful or failed. So this means it is not possible for it to forward the request to a second DNS server if it has already been rejected by the first.