How do I open the same file side by side in a single Visual Studio Code session
In the top right of your screen there should be a split editor button that looks like this: 
That should do the trick.
The default keyboard shortcut (on windows) is Ctrl+\
In Command Palette
(Ctrl or ⌘+Shift+P)
Type split e Enter
(Select View: Split Editor)
In sidebar
Alt+Left-click the file
In editor
Click the Split Editor button (
) in the upper right corner.