Remember this? Turns out there was more to this rumour than we thought. As Steven Troughton-Smith notes (and yes, you can trust him):
So... just in case there was any doubt left... iOS 8's SpringBoard has code to run two apps side-by-side. 1/4 size, 1/2 size, or 3/4 size
With Apple pushing developers to use Auto Layout as hard as they can, we can pretty much assume that yes, multiwindow is coming to iOS. Note, though, that this is not a Windows 8 or Samsung or whatever feature - multiwindow is as old as the graphical user interface itself. It will be a great addition to future iOS releases, and I can't wait until Android implements multiwindow as well.