Nginx – Configure nginx to forward external port to same internal port

I'm running TileMill, which is listening to port 20008 and 20009, from localhost. I want 20009 to be only accessed through nginx on port 80 (with simple auth). I want 20008 to be accessible "directly" from the outside.

server {
   listen 80;
   server_name localhost;
   location / {
        proxy_set_header Host $http_host;
        proxy_pass http://127.0.0.1:20009;
        auth_basic "Restricted";
        auth_basic_user_file htpasswd;
    }
}
server {
   listen 20008;
   server_name localhost;
   location / {
        proxy_set_header Host $http_host;
        proxy_pass http://127.0.0.1:20008;
    }
}

The obvious problem is that nginx can't listen on port 20008 – it's already being used by TileMill.

Restarting nginx: nginx: [emerg] bind() to 0.0.0.0:20008 failed (98: Address already in use)
nginx: [emerg] bind() to 0.0.0.0:20008 failed (98: Address already in use)

Is there a way where nginx can listen externally without conflicting with the other service listening internally? (Changing localhost to the server's external IP didn't work.)