All iPads running iOS 9 support slide Over, where you can slide a second app out to take up 1/3 of the screen (while other app shifts to background). As such it is not 'multitasking', just a way to peek without having to fully switch. I haven't discovered a way to 'promote' the slide out app to full screen, unless the 'slid onto' app supports split screen. That's a little weird. And you can't Slide Out from left side.
For split screen you need an iPad Air 2 or the new iPad mini 4, or soon, iPad Pro. Both apps are foreground and respond to input.
Apps have to implement these features to serve as a slide out app, or to run in split screen. iBooks, for example, supports both.
But only one instance of a given app can run in Split Screen (or run, period). So you can't read two books in iBooks at the same time. The two apps have to be different apps.
When Amazon gets around to updating Kindle app, you will be able to have iBooks and Kindle, etc., but right now I think iBooks is the only reading app that supports these things.
Of course there's no reason a reading app could not implement split window, for viewing two books at the same time, but for some reason nobody has bothered to do that. Or maybe Apple UIG discourages that.
Then there is Picture-in-Picture (again only on iPad Air2, iPad Mini 4, iPad Pro), which lets you run a video or FaceTime session in a PIP window while you do something else (including Slide Out or Split Screen). Again app in question has to implement it to run in the PIP window. The Movie app does this but haven't found any others yet.
I'm surprised there aren't more apps ready to do these things on day one, but maybe the apps are working their way through approval cycle. Microsoft Office apps do 'multitask' with latest update. Good for them!
To me, it is a little underwhelming so far, but more apps need to update to draw any conclusions.
http://www.macrumors.com/2015/09/16/...itasking-ipad/