How to switch or close the new split Terminal pane? I stumbled across the CMD-D hot key which splits the Terminal into 2 panes.  But I can't find documentation anywhere on how to close the new pane or even switch the focus to it so I can use it. What are the other commands?
The idea behind splitting is that it allows you to keep a certain part of the shell buffer displayed while continuing to enter new commands. So only the lowest split does allow keyboard input. To position the view on the shell buffer use the scroll bar.

You can un-split by pressing Shift-Cmd-D.