Create two side by side windows and enter Follow mode.
Execute this command to display as much as possible of the text
in the selected window. All other windows, in the current
frame, are deleted and the selected window is split in two
side-by-side windows. Follow mode is activated, hence the
two windows always will display two successive pages.
(If one window is moved, the other one will follow.)
If ARG is positive, the leftmost window is selected. If negative,
the rightmost is selected. If ARG is nil, the leftmost window is
selected if the original window is the first one in the frame. (fn &optional ARG)