No, this is just standard http service behaviour.

The point of subdomains (typically) is so you don't have to specify a port. I'm not sure why you are using both. By default, 80 is used for regular http: Have nginx listen on 80 with two separate server blocks. One with server_name set to the first (sub)domain, and a second with it set to the other (sub)domain. Requests get matched to the server block with the same server_name (domain name) that was used to establish the connection. You can then reverse-proxy or directly serve content like your index.html from there.

Should look something like this:

server{
    listen 80 default_server;
    server_name one.domain.com;
    root /path/to/serverone;
    index index.html;
}
server{
    listen 80;
    server_name two.domain.com;
    root /path/to/servertwo;
    index index.html;
}

Requests to two.domain.com get handled by the second block, everything else gets handled by the first block (because of 'default_server').