I don’t know the answer, so I’ll just post this screed from the terrible film “Psychos in Love”.

I hate grapes! I can’t stand grapes! I loathe grapes! All kinds of grapes! I hate purple grapes! I hate green grapes! I hate grapes with seeds! I hate grapes without seeds! I hate them peeled and non-peeled! I hate grapes in bunches, one at a time, or in groups of twos and threes! I fucking hate grapes!

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.