In the Settings for Minecraft Windows 10 Edition under Video there are options concerning Split Screen, I've never been able to use splitscreen so I'm wondering if anyone can tell me if I'm doing something wrong or if Splitscreen just isn't there.
Console Minecraft uses split screen if you play with multiple players on one console. Since Windows 10 edition is practically the same game, it also has this option.
I think Windows 10 edition also allows to play with USB controllers. So you could for example play with one person on keyboard and mouse, three on controllers and a split screen.
I did not confirm that splitscreen actually works, it likely doesn't. There is this bug report for it.
Split-screen has not been added to windows 10 minecraft.
There is an open feedback post on the Official Feedback website where community members can vote for the feature to be added.
If you want this feature to be added, you should vote for it there.