View Two Websites Side-By-Side With Safari Split View in iOS 10

First split screen only works in landscape mode, not portrait. There three ways to achieve this. 

Open a new tab is split screen

Tap and hold the the tab management icon in the upper right. You will see an option to Open Split View. Tap it to create a blank Safari window. 

Open a link in split view

Tap and hold a link on a web page and select Open in Split View

Drag a tab to open in Split View. 

Tap and hold a tab, then drag it to the side of the screen you want it to open in.