Or, as long as it's enabled in Keyboard settings, you can simply tap and hold on the small keyboard button at the bottom right of the keyboard. You'll get dock/undock and split/un-split options. Or you can tap and drag the button up. Apple supplied a lot of ways to switch between them.
My problem is that like the FaceBook app, most developers have not figured out how to handle a keyboard that suddenly ends up in the middle of the screen, often covering the text you are trying to type. Though FaceBook at least moves the entry field, even if it loses the send button.
(notice how I so cleverly came back to the topic)
(ouch, strained something trying to pat myself on the back.)