How can I fix the Dual Monitor Fullscreen issue without updating to Maverick Here's the problem:
When one app (for instance YoutTube or a game) goes fullscreen on monitor A, monitor B slides right, into a new gray "desktop" (like the ones one can add in the top right corner via the F3 button) that doesn't allow any apps to be in it. Because of this, one screen becomes useless when the other has a fullscreen app running.

I see many others having the same problem, but I can't seem to find a way to fix it. 
I'm not interested in app specific fixes. 

Thanks.
This was fixed in Mavericks as you are aware, but I don't know of any canonical way to fix this for apps in prior versions of OS X, and even app specific fixes don't work properly.

Whilst you said without updating to Mavericks, this really would be the best option since it is the 'official' fix to a core part of OS X.