If you want to host multiple domains on one droplet, you'd have to create additional nginx/unicorns configs for each domain, you'd also have to launch a separate unicorn process that listens on another port and not 8080 for each domain so you will need a separate service to launch the second process.

See https://www.digitalocean.com/community/articles/how-to-1-click-install-ruby-on-rails-on-ubuntu-12-10-with-digitalocean