When the Galaxy Note 10.1 was released, it had a feature where you can snap two apps side by side. Unfortunately, it only worked with a few specific Samsung apps, nothing more. The Galaxy Note II improved on that with a bigger set of compatible apps, making the feature far more useful. However, you still couldn’t use any app you wanted. But who doubted that the amazing dev community wouldn’t get around that?
An XDA user has created a mod for Galaxy Note II custom ROMs that allows you to use any app for the Multi Window feature. You have to be rooted to flash this file in recovery, and you must be running a deodexed ROM (sorry stock users, no fun for you). You just flash the mod associated with your build and start using all your apps in Multi Window!
Of course, perform this procedure at your own risk. None of us are responsible for any possible damages to your phone. Make sure not to flash the wrong zip for the wrong build. And good luck!