@drsebro @rf Different nginx config files all live in the same space so what you need is to ensure that stuff outside of the server {} block is only present once globally (e.g. map and proxy_cache_path). Then for the upstream {} blocks, you gotta name them different things, like upstream backend_foo {} and upstream backend_bar {} etc.
@Gargron thank you for your quick answer. Unfortunately it looks like I need a straight forward instruction :) I can already start Nginx with some adjustments suggested by you, but there is still something wrong configured. My second instance is showing partially content of the first one.
Should I ask this question on https://github.com/tootsuite/mastodon ?