By default, Windows + arrow key left or right turn a selected window snap to the screen’s right or left. If you press Window + Up or Down, it will resize the selected window to a focused window or full-screen window size. This all works great for monitors positioned horizontally, but for the vertical monitor, you most likely want the screen to split your app top or bottom of the screen instead of left and right.
Without any modification or addon to Windows, there is no magic to split the window evenly between two apps and share half of the entire screen on a vertically positioned monitor. Thanks to Microsoft’s own PowerToys, one of the tweaks allows you to position any window to a predefined layout.
Install PowerToys, launch the app and find the section named FancyZones. This is where you can enable the behaviour to extend the Windows + Up/Down key to split an app into the top and bottom of a vertical screen.
Under the Window Behaviour make sure you select “Win + Up/Down/Left/Right to move windows based on relative position.“
Then, edit the screen, select the monitor stacked vertically, pick the layout, and adjust the columns or rows.
Here I’ve changed the first vertical monitor to have two rows. Now try and press Windows keys with arrows Left/Right and then Up/Down. You should be able to split your windowed app into two rows where they split evenly across the monitor.
Last but not least, if you don’t want any special FancyZones apply on other monitor, just select the first no layout option. With that you can still manage your windowed application with the default Window + Left/Right to split the app. Hope this helps with your Windows 10 experience.