Q: split screen on iPhone 6s Plus?

Hello klai_apple,

Thanks for using Apple Support Communities.

Split View is not a feature on iPhones.  You will need to have an iPad Pro, iPad Air 2, or iPad mini 4 in order to use Split View on an iOS device.

Use Split View 

With Split View, you can open and use two apps at the same time. Open an app, then tap or drag the app divider to start using both apps. Slide down from the top of the screen to change the app on the right. If you want to close one of the apps, slide the divider over the app that you want to close.

You can use Split View with multiple apps, like Safari, Notes, Photos, Calendar, and more. Split View isn't available with all apps.

You can use Split View with iPad Pro, iPad Air 2 or iPad mini 4.

About Multitasking on your iPhone, iPad, and iPod touch

Take care.