Unless you insist on a sub-folder install, the best and easiest way for most people is to just run two droplets. To run them both, you’ll need 2 GB, so there’s no cost savings. If your traffic is such that you need a 4GB droplet, then it might make sense to use a single droplet.
If you do want to do the single-droplet approach, there are howto documents here for “running discourse with another web server”.