How to use Slide Over and Split View on iPad
Source: iMore Split View lets you have two persistent apps sitting side by side on your iPad. Depending on the screen size of your iPad, those two apps may be displayed as Compact (iPhone UI) or Regular (iPad UI) next to each other. On certain iPads, you can also resize Split View panes for a 50-50, 25-75 or 75-25 split in a horizontal orientation; when holding the tablet vertically, you'll only have the 25-75 or 75-25 options. 50-50: Each app takes up the exact same real estate on the iPad. 25-75: The app on the left takes up just 25% of the screen, with the right app taking up 75%.
Source: imore.com