Cant setup subdomain (address in use)?

I am not seting up my nginx virtial host blocks up properly, but I don’t get how to serve a subdomain on port:80 and the main domain on port:80?

I tried creating multiple virtual hosts, but I was getting an error with server_names_hash_bucket_size 64; I had to uncomment that out inside nginx.conf to get it work and at this point I already went ahead an merged both virtual hosts into the default file.

server { listen 80;

root /var/www/example.com/public_html/public;
index index.html index.htm;

# Make site accessible from http://example.com
server_name example.com;

}

server { listen 80;

root /var/www/demo.example.com/public_html;
index index.html index.htm;

# Make site accessible from http://demo.example.com
server_name demo.example.com;

}

Obviously I am pointing both to port:80 but I am not sure how to configure it so they both work on port:80.

I followed https://www.digitalocean.com/community/articles/how-to-set-up-nginx-virtual-hosts-server-blocks-on-ubuntu-12-04-lts--3

But I am missing something.