As long as your link between the applications share the session ID, the solution is simple - set the cookie name the same across your applications.
Shared components -> Authentication Schemes -> Edit current scheme -> Session cookie attributes -> Cookie Name
|Set same cookie name across applications|
|Possible application map|
ie - if you log into application B using DB accounts, you can still hop across to application A.
The drawback I've found is that post-authentication will only fire during your initial login (in the relevant application). This means you need to be aware how to handle and share your authorisation mechanisms.
Divide and conquer!
If you're on 18.1 (5.2), you may be interested in the Social Login feature. Dimitri has a detailed post.
If you're on 18.2, you'll need to set Type to 'Custom', or (carefully) consider the Workspace sharing option.
|APEX 18.2 options|
(Oct18) I'm currently writing a presentation on this topic, stay tuned.