Is there any way and have there been any examples of split-screening two apps on iPad so, for example, I can be scrolling through a PDF or PPT and simultaneously be typing notes about it in a text editor with an external keyboard?
You can use iOS 9's Split View.
With Split View, you can open and use two apps at the same time. Open an app, then tap or drag the app divider to start using both apps. Slide down from the top of the screen to change the app on the right. If you want to close one of the apps, slide the divider over the app that you want to close.
This requires iPad Pro, iPad Air 2, or iPad mini 4. For more information, see: