I don’t think anyone is arguing against dual DNS servers. The distinction being made is that a second DNS server is not a fallback. Most newbies think “secondary” means it will only be used when the primary is unavailable. That’s not true. A client is just as likely to use a secondary DNS as a primary. If only one DNS uses pihole, then the secondary will serve ads because it’s just calling the upstream DNS resolver.
Personally, I accomplished what OP is talking about with two rPis. First serves DCHP from 192.168.1.10 to .100, second serves .101 to .250. I send the two piholes as primary and secondary DNS. I also use Unbound as the upstream, but that is just personal preference.
I don’t think anyone is arguing against dual DNS servers. The distinction being made is that a second DNS server is not a fallback. Most newbies think “secondary” means it will only be used when the primary is unavailable. That’s not true. A client is just as likely to use a secondary DNS as a primary. If only one DNS uses pihole, then the secondary will serve ads because it’s just calling the upstream DNS resolver.
Personally, I accomplished what OP is talking about with two rPis. First serves DCHP from 192.168.1.10 to .100, second serves .101 to .250. I send the two piholes as primary and secondary DNS. I also use Unbound as the upstream, but that is just personal preference.