I have one project like this, but so far I have only ported the admin console sections to CI4 which already used a separate auth, so I have yet to cross the bridge of a common auth. Myth:Auth is nice that it has a lot of features but at the end of the day an active session just means $_SESSION['logged_in'] is the current user ID, so that should be pretty easy to hijack in CI3. No other flags I can think of for passing back and forth between the two, just make sure cache (etc) prefixes don't overlap for non-compatible components.