Is what possible? Failover? Simply no. Assigning different ISPs' DNS IP addresses as preferred DNS server to different computers has no sense and no use while gateway address is intact.
A workaround can be in a way where you set your DHCP server to distribute two gateways and two DNS servers. I remember a registry entry can make the computer act as a IP SLA monitor to switch between gateways, but cant remember if supported in XP.
This would largely depend on your ISP's DNS settings. Most ISPs allow lookups (or recursive lookups) only to their clients (as in IP Addresses assigned to them). If you failover to ISPB and the clients machine has First ISP's DNS server assigned to it, this will create issues. Just assign two DNS servers to each machine, putting the primary ISP (the one mostly active) as the Primary DNS also. If it goes down Windows will automatically fallback to the second server.
You can also assign One Public DNS Server (not belonging to any of your ISPs).
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...