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