The short answer is that it is possible. The long answer is that you'll likely have to adjust one or the other's authentication methods to use the other scripts functions. You would also need to alter the session handling sections of both codes so that a user will be logged into both when they log into one of the scripts. This can be extremely complicated and will require a break check every time either application is altered.
A possible third option is to add a hook into the user creation methods of both scripts and add the user to the alternate application at the same time. You would also need to update the password for the alternate application at the same time as well. The big problem with this method is that users would still need to log into both applications independently.
/author/test/
to/forums/users/test
, or visa-versa? Or do they each have unique data that you want to merge into a single page?