You can't close it one at a time when it's in a split view pair, it only lets you close the whole pair together. If you just want to kill one of them, first you have to grab the middle divider and push it all the way to one side or the other to make either app full screen. The other app becomes backgrounded.
Once they're separate, you can close them from the app switcher by swiping them off the top of the same, same as an iPhone.
