Terminal: Switch between panes after vertical split
From this answer:


  You're misinterpreting the feature. It's not meant for two separate terminals. It's intended to allow a user to see two different view points in the same terminal. For instance, if you have 3000 files in a directory, and you perform an ls command, that output is going to be very long.
  
  If you use the split pane, you can scroll through that long output without having to flip back and forth, possibly losing your place along the way.
  
  If you want two terminals, use tabs, or separate windows.




Alternatively, you can also install and use a widely used 3rd party Terminal emulator called iTerm2. iTerm2 has various features including the ability to create Split Panes, with support for keyboard shortcuts.