Yepp, some newer ROMs have atleast a Split-Screen-View implemented.
Not sure if there are 4.2-based ROMs with it, but 4.3-PAC&Omni-Builds should support it.:fingers-crossed:
Sadly Android, or the feature is not yet fully capable of true Multi-Tasking, but I guess this belongs to the System-UI and the button-mapping of those buttons (Back, Home, etc.) the the "selected app". So if you have a Youtube-Video running on one side and click onto the side where for example google play is running (to select an app to install, or whatever), it stops playing the Video in the YT-App, because it isn't "the selected app", while it technically should be capable of still let the video running (not sure, but I think I heard google implemented a feature like "keep running")...