ediff-merge-split-window-function

ediff-merge-split-window-function is a variable defined in `ediff-wind.el'.
Its value is

split-window-horizontally

• This variable may be risky if used as a file-local variable.

Documentation:
The function used to split the main window between buffer-A and buffer-B.
You can set it to a vertical split instead of the default horizontal split
by setting this variable to `split-window-vertically'.
You can also have your own function to do fancy splits.
This variable has no effect when buffer-A/B/C are shown in different frames.
In this case, Ediff will use those frames to display these buffers.

You can customize this variable.