[willis936]

Both. Have your router (optionally all clients) point to each DNS server. You can go further by keeping the pi holes synchronized:

https://github.com/vmstan/gravity-sync

Since some hostile clients (such as TVs) have hard coded DNS, it is necessary to forward all port 53 and 853 traffic to a pi hole. This is easy enough with NAT redirection rules in the router, even with two pis.

https://www.myhelpfulguides.com/2018/07/30/redirect-hard-cod...