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!