Just wanted to add onto your comment for clarity for others, the multiple servers are not redundancy so much as first come first serve, which is why your comment of multiple pi-holes is important.

If you were to list a pihole and say Google DNS as primary and secondary respectively, you may have some DNS queries responded to by Google. Negating the point of having a pi-hole or similar DNS service locally.

A secondary can be a docker container, another physical pi-hole (even a zero-w, which I personally don’t recommend being your only way to manage DNS, but is fine when you just need to do some maintenance on the primary).