Nginx...two domains, same sites?

You just put both in the server_name line and you're good to go.

In what sense is DuckDNS unreliable?

I switched from duckdns about a year ago as it failed to resolve the addresses for my jellyfin server. I ended up buying a domain from cloudflare for 3 years for about $4, and I self-hosted ddns updater to automatically grab the dynamic ip, and set it to a subdomain.

As for your nginx config, I'd imagine you could make 2 separate config files in sites-enabled that are nearly identical, but listen for different domains. Something like this:

#config file 1 
server {
    listen 80;
    server_name example_a.com;

    location / {
        return 301 http://example_c.com$request_uri;
	#or use an ip instead of example_c.com
    }
}

#config file 2
server {
    listen 80;
    server_name example_b.com;

    location / {
        return 301 http://example_c.com$request_uri;
	#or use an ip instead of example_c.com
    }
}


#Or use "proxy_pass http://example_c.com;" in the location tag instead of "return 301..." if you want to reverse proxy the traffic

I liked FreeDNS when I used them, but that was something like 10 years ago. Could be worth looking into.

I'm currently moving from duckdns to desec.io — with the hope of it being more reliable.

Would recommend OVH for both domain name and DNS, they have DNS APIs so you can get certificates from lets encrypt with ease. Also you could update your zone when your home IP changes

How is duckdns unreliable? I use it just to have wireguard access, been using it for years. Just curious about your issues...

This should be possible, in nginx you would just have near identical entries that deliver the same content. The service itself sometimes takes a domain to build internal links etc, and those usually only take one.