In order to serve both sites from the same Droplet, you will need to configure both the Express app and Ghost to listen on separate port with Nginx directing request to the correct destination.

You Nginx config would look something like:

server {
    listen       80;
    server_name  localhost;

    location ^~ / {
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;

        proxy_pass http://127.0.0.1:3000;
        proxy_redirect off;
    }

    location ^~ /blog {
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;

        proxy_pass http://127.0.0.1:2368;
        proxy_redirect off;
    }
}

For more information, check out these tutorials:

• How To Set Up Nginx Server Blocks (Virtual Hosts) on Ubuntu 16.04
• How To Set Up a Node.js Application for Production on Ubuntu 16.04