Open the context menu on a document tab by right-clicking on it.
Select Split Vertically.
You will now have two columns to place document tabs into:
If you want to assign a hotkey to the action, do the following:
Open the Settings (Ctrl+Alt+S).
Search for Split Vertically (or Split Horizontally, if that’s what you’re after) and select the Keymap in the IDE Settings.
Open the context menu for the action in question and select Add Keyboard Shortcut.
Assign the desired hotkey. In this example, I chose Ctrl+Shift+P,V:
Here we can see the result of assigning a hotkey to both the vertical and horizontal split action: