Setting up a custom domain with pages instructs users to set up a DNS A Record to point to 204.232.175.78 in order to use a custom domain with their GitHub pages.
However, at 101domain.com, it tells me: You need to specify atleast 2 name servers. 101domain won’t let me specify the same name server twice.
Thus, is there a second DNS server for GitHub pages with custom domains? Most hosts offer between 4 or 6 DNS servers
You’re specifying the wrong thing.
You need to set an A record with an existing DNS provider; GitHub does not give you a nameserver.