See also bug 32369. A login via OPAC should imo not allow you to access staff too when you share the same domain (and cookies) between them.
I think in a way this is a requirement for the older bug 15428. When we have different sessions, we can also implement different timeouts if I am not mistaken.
Something else to keep in mind is cookie authentication for the REST API interface via the staff interface and OPAC. Maybe this is an opportunity to switch away from "CGISESSID" to more Koha-specific cookie names. There are over 200 references to CGISESSID in the code, although to lessen the work we could just change the cookie name for the OPAC. That would involve fewer script changes. Auth.pm would need an update at a minimum though. If someone is willing to do the work, I suppose there's no reason not to...