Function: 2C-two-columns

Split current window vertically for two-column editing.
When called the first time, associates a buffer with the current
buffer in two-column minor mode (use C-h m once in the mode,
for details.). It runs `2C-other-buffer-hook' in the new buffer.
When called again, restores the screen layout with the current buffer
first and the associated buffer to its right.
(fn &optional BUFFER)