New 21.5-inch Retina 4K models expected.
Video Showing How Apple's Unfinished Split Screen iPad Multitasking Works
The feature was first rumored in March for introduction in iOS 8, but was pulled because it was not ready in time for WWDC. However, it was discovered just a couple of days ago that iOS 8's internal SpringBoard includes code that allows apps to run side-by-side.
In the video, the three sizes are shown (1/3, 1/2 and 3/4) when using two fingers to scroll to the side. However, it's not evident how to open a second app next to the already opened app, likely because the feature is still being developed.
Split-screen multitasking may initially be limited to the iPad Air, with support for smaller devices, like the iPad Mini and rumored 5.5-inch iPhone 6, at a later date. The feature could also make it to Apple's rumored 12.9-inch "iPad Pro" that may or may not be in development.