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.