FWIW I manage two Discourse installs linked to WordPress sites w/ the WP Discourse plugin, but on separate servers, works great. I started out just knowing WordPress so I can see why it feels like it should maybe be simpler to just use one server for both, but I think actually not the case.
If the main concern is cost, keep in mind a dedicated server for Discourse on Digital Ocean starts at just $5 / month. That’s what I opted to use for my own small side project which doesn’t make any revenue, but the way I’d think about it, if it saves even a few hours of your own time per year vs. some more complex setup, it’s worth the relatively small cost!