I concur, I’d keep them separate. It can be easier to set up other tools like CI, dependency updaters (dependabot, for example)… and to @stephencattaneo’s point I think it’s definitely better to create a strong mental separation between the two. I used a backend/frontend monorepo at my last company and it didn’t really gain us much of anything (vs an ember monorepo which can be much less annoying to manage).
Yes the two are separate. Keep em separate. IMHO attempting to build ember via a rubygem is a nightmare (I’m sure the folks who built Discuss can attest).
However, that doesn’t mean you need two repos. You could have an umbrella like repo that has two directories in it one for the rails app and one for the ember app.
But if that setup is more complicated then you want for a greenfield Rails app you could also just host the ember app on a static web hosting service (like S3 or Dropbox) and then have your Rails app send CORS headers for that domain.