If you are familiar with reverse proxy configuration, you don’t need the second IP.
You can run both Discourse and Wordpress listening on unix sockets (or higher ports) and run a reverse proxy software in front (I recommend Caddy) to serve on the same IP your blog.example.com and forum.example.com.
That said, if you aren’t familiar with this type of setup, running each in it’s own VPS is much easier as you can just follow the vast documentation for each.