windows horizontal split screen

Natively, it seems, you can't.

You can work around this using the left/right/up/down snapping capabilities plus a final use of your mouse.

Open your two windows
Bring focus to one of your windows
Press Win + Up Arrow to maximize the window
Press Win + Right Arrow to snap the window to the right half of the screen
Press Win + Down Arrow to snap the window to the bottom right quarter of the screen
Repeat 1 - 5 with your second window. Note: on step 5 press Win + Up Key to position the window to the top right quarter of the screen
Use your mouse to extend both windows +
+ so that they extend the width of your screen. 
Note: I have noticed that, even with multiple screens, Windows 10, 
whilst extending a window horizontally, will snap the window to the full width of the screen.
If you need to extend past the screen (with multipe monitors, for example) then you can continue moving your mouse and it will un-snap and continue extending.