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forgive the ignorance, but is that a consequence of Apple's dimensional choices? I've seen split screen work on both the S5 and the Note. I've never really had a use for it but it does work. Just tried it on my S5 and the screens are re-sizable. It just seems a vertical stack on the Plus should be possible, but what do I know.

It's a consequence of developers focusing on just one screen (before the iPhone 5). If an app didn't support landscape for the iPhone, then the app developers never needed to worry about screen sizes less than 480 points.

Android developers have to worry about all of the different screensizes from day one because Android lived on many different screensizes.
 
Upppsssssss It does works, but only with Video app, no with YouTube and so on........

The developers should build support for the features like Picture-in-picture (PIP), Slide over and Split view (Split view: only iPad Air 2 and iPad mini4).

The official YouTube app doesn't support it yet but there is this unofficial YouTube app called 'YouPlayer' by Homegrown Software Ltd.

See a complete list of apps in this wiki topic in the iOS9 area.
 
As was pointed out, Slide Over is not multitasking... One cool thing I've discovered, however, that works at least on my iPad Pro with apps that support Split Screen, is that I can "slide out" an app that supports Split Screen. At this point, the app in the main (left) part of the screen dims as expected. However, if I now drag the Slide Over app into Split Screen mode and then drag it back to its Slide Over position, I now have full multitasking available with both apps. I've found this particularly useful for Messages. I can participate in a Messages conversation using only 1/3 the screen, while still being able to fully interact with my main app in the left 2/3. Although I don't use portrait mode very often, it even works in this mode just by "trying" to slide the Slide Over app towards the center and letting it spring back to its 1/3 mode (because Split Screen does not appear to work for me in portrait mode). Now, even in portrait, I have full multitasking with a 2/3 - 1/3 configuration.
 
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