How to split app screen on my Mac?
The supported Apple way of using split-screen for El Capitan onwards is documented at https://support.apple.com/en-gb/HT204948 in the section titled 'Use two apps in Split View'. It works nicely for apps that support it. The most up-to-date docs for Mojave are at https://support.apple.com/en-gb/guide/mac-help/use-apps-in-split-view-mchl4fbe2921/mac

You can hold and drag the green maximise button of a window to one side of the screen to 'lock' it in position, then select another window to fill the other side. The same functionality is also available in Mission Control (ctrl-up arrow).

I'm not aware of a command that will do what you require, but it's possible using the GUI gestures.