Is there a simple Load Balancer app for development environment on Windows?

You can use use Nginx

Install nginx and add config file:

http {
  upstream myproject {
    server 127.0.0.1:8000;
    server 127.0.0.1:8001;
  }

  server {
    listen 80;
    server_name localhost;
    location / {
      proxy_pass http://myproject;
    }
  }
}

You could use Apache HTTP Server with the mod_proxy_balancer module.