There is a Reddit Post (https://www.reddit.com/r/ios/comments/14k2kq1/how_to_configure_a_secondary_doh_dns_to_my_iphone/) about a secondary DoH DNS setting in iPhone.
But it seems that the DNSSettings doesn't support this.
DNSSettings (Required) A dictionary that defines a configuration for an encrypted DNS server.
It can only specify "an" encrypted DNS server.
Maybe I missed something. Is there a way to create a profile to support a primary and a secondary DNS-over-HTTPS DNS servers ?
I guess, however, iOS doesn't support 2 DoH DNS servers as the traditional DNS servers. Then Apple could consider to support primary and secondary DoH DNS.