Hi webbuster,
Yes it makes sense and it's possible. It depends on the other software you choose and if they have hooks or not.
It should be something like this :
OSClass' user management <-- custom code --> phpBB/SMF's user management
OSClass has enough hooks to control both, OSClass and other software user management. I don't know if the other way could be possible.
Let me explain, you have to software call them OSClass and softX, they can not share user's table, so here is the problem
You register in OSClass -> you are not register in softX
Then you make a plugin (custom code), osclass has some hooks (user_registerd, user_login,...)
You registered in OSClass -> your plugin modify softX databse -> you ARE registered in softX
You registered in softX -> you are not registered in OSClass
Then you should :
A.- create a plugin (if they have a plugin system)
B.- modify core files (updates of softX will be difficult if not impossible)
This also happen at modifying your information (for example, if you change your email in one of the two softwares).
The best solution is to have both plugins, so it doesn't mind if your user registered/modify their data/log in in any of the softwares