My guess is that you can run multiple engines (Discourse installations) on top of the same database (so you need a separate database). I would only run 1 sidekiq, though. Otherwise two of them would start fighting for the jobs. Other than that I think Discourse is a stateless app, so it should not matter how many of them you are running.
Now a question to the root of the whole issue: Why? And is it not against SEO rules? I think Google disliked if it found the same content on multiple sites.