Settings | Home Screen & Dock | Multitasking | Allow Multiple Apps

No idea if the same setting exists on iPhone or if it disabled split screen through.

The whole feature is so badly designed on iPad. I was forever detaching a Safari page and ending up with two Safari apps. So tapping the Safari icon wouldn't take me back, it only takes you to the detached page, and you think you have "lost" all the other pages. You need to close the detached page (none of that would be obvious to a novice). Also getting a page to detach into a popup by mistake, and then not being able to get rid of it (I still don't know the correct swiping motions!)