Split-screen multitasking on iOS has been hiding in plain sight
Third-party keyboards are one of the tasks in multitasking, even if it doesn't feel like it
I’m very curious to see how you get into (and back out of) this mode, though. It’s easy to say you want to see two apps side-by-side on the iPad. It’s not easy at all to implement such a feature without losing the wonderful can’t get lost/can’t get confused about where you are or how you got there simplicity of the iPad today.

The third-party keyboard is the second app in the split-screen. It's just a vertical split instead of a horizontal one, as is more common on the desktop, and as Microsoft does it on the Surface. Look at the Riffsy keyboard for finding and sharing GIF's or other third-party keyboards: many of them are just mini-apps.

The beauty of it all is that they did it without compromising the "simplicity of [iOS] today", as Gruber put it. When you think about the most common reason to have two apps open side-by-side — browsing for content in one app to copy/paste into the other — it makes perfect sense to have done multitasking this way, because that's exactly what advanced keyboards are useful for. It just doesn't feel like multitasking.
Neat-o.
¯\_(ツ)_/¯