One option would be to use DO private networking. Where one droplet would host your application server and the other would hold the database. Or maybe have your master droplet remote mount directories from your slave droplet. In both examples, you would probably want all of those resources only directly accessible behind the firewall. NOTE: private networking need to be located in the same data center, last time I looked.

Another tool to use with multiple droplets is to use DNS to seperate your URI’s, such as one droplet serving your http or https URIs, and another droplet being your ftp resource. Even though both droplets would be directly accessible, you can still use private networking between your droplets to have your server apps talk to your privately networked database.

Why not just use Load Balancing?