WinSCP / DNS

I have a primary DNS server which undergoes a weekly reboot cylce, during the hours downtime, WinSCP is unable to resolve hostnames.

Example NSLOOKUP - Windows PC
The primary server is the only DNS server that NSLOOKUP sends all queries to unless you tell it otherwise, meaning I have to tell NSLOOKUP to use another server.

Is WinSCP able to use the secondary/tertiary DNS servers in sequence if the primary DNS server times out?

WinSCP does not talk to any DNS. It uses a system name resolution API.