2C-merge is an interactive compiled Lisp function in `
Merges the associated buffer with the current buffer.
They get merged at the column, which is the value of `2C-window-width',
i.e. usually at the vertical window separator. This separator gets
replaced with white space. Beyond that the value of `2C-separator' gets
inserted on merged lines. The two columns are thus pasted side by side,
in a single text. If the other buffer is not displayed to the left of
this one, then this one becomes the left column.
If you want `2C-separator' on empty lines in the second column,
you should put just one space in them. In the final result, you can strip
off trailing spaces with M-x delete-trailing-whitespace.