For years now I've been using Ctrl+Alt+T to open a new terminal window. Just now for the first time it took my existing window and created two panes within it:
^^ New terminal session with
~/.bashrc start up commands showing date, time and system information. ie No commands run yet
I must have hit a different key combination as I just tried it again double checking as I pressed Ctrl+Alt+T and a new window as normal.
As I might want to utilize this feature in the future, What key combination did I use?