Here’s a function that will change a pair of vertical windows to a pair of horizontal windows:
(defun 2-windows-vertical-to-horizontal ()
(let ((buffers (mapcar 'window-buffer (window-list))))
(when (= 2 (length buffers))
(delete-other-windows)
(set-window-buffer (split-window-horizontally) (cadr buffers)))))
To do this automatically on startup, add this function to emacs-startup-hook
:
(add-hook 'emacs-startup-hook '2-windows-vertical-to-horizontal)