PostgreSQL - tuning multiple clusters on server
You should probably cut max_connections by a lot.  I don't know why it would recommend such a high setting in the first place.  I'd also cut shared_buffers down to the default (128MB) or maybe lower, as PostgreSQL runs on top of the OS file cache and with two separate instances it would probably be best to let the OS decide how to use the available memory, both between instances and for other uses.

I'd leave effective_cache_size and effective_io_concurrency alone.  Those are short term issues, and postgresql makes no effort to partition them between connections even within an instance.  It is possible that two connections on separate instances want to make use of those "facilities" at the exact same time and interfere with each other, but it is also possible that two connections on the same instance would.  So the mere fact that they are being shared is not a reason to lower them.  These setting don't do much for transaction processing anyway, they are mostly important for large analytics-type queries.