Can I have two apps running split screen on an iPad? 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'd have to have one app that does both the displaying and input, and I know of no apps like that.

There is no provision for split-screening apps, even iPhone apps. The best you'll be able to do is fast app switching.