at the moment there are 3 shortcuts; 2 shortcuts would be better with do/undo; so, to split window with one shortcut would be the same shortcut to undo split the window <br> even better would be a 2x if-undo for each shortcut, like this:

shortcut for split up and down: if window is splitted left and right, then: undo; if window is splitted up and down, then: undo, else: split up and down

shortcut for split left and right: if window is splitted up and down, then: undo; if window is splitted left and right, then: undo, else: split left and right