I would like to open the file
foo.bar twice (or more) in emacs, so I can edit two different parts of it simultaneously. Is it possible? Probably the better question is, how to do it? Is there a way to open each instance in its own buffer/frame?
You can open the same buffer in more than one window (which can be spread amongst different frames), but it's not very convenient. Each window has its own point, but they all share the mark, the file mode, the narrowing and other characteristics, because apart from the point pretty much all characteristics are tied with the buffer. Also, if you happen to visit another buffer in one window, you'll lose your place in the file.
You can make an indirect buffer which has its own point, mark, modes and so on, but the same content as the original buffer (and saving either buffer writes to the same file). To create a second buffer that is a clone of the current buffer, run
M-x clone-indirect-buffer RET. To open that second buffer in a different window, you can type
C-x 4 c.
Split screen mode: where ctrl+x means press and hold ctrl key and type an x. Then type the number following.
ctrl+x 2 (horizontal split)
ctrl+x 3 (vertical split)
Then you can scroll them independently on the same file or open another buffer if you wish in one of them.
To return to a single view type
If you wish you can split each screen (section) as many times as required. Select the screen you want to split first and then perform a horizontal or vertical split on it.
Emacs is biased to panes (windows), not frames. It is often desirable to open the same buffer in another frame, not just another window in the same frame. But
C-x 5 c is unbound by default.
This code define the missing
(global-set-key [?\C-x ?5 ?c] '(lambda(newname display-flag) "Like `clone-indirect-buffer-other-window' but display in another frame." (interactive (progn (if (get major-mode 'no-clone-indirect) (error "Cannot indirectly clone a buffer in %s mode" mode-name)) (list (if current-prefix-arg (read-buffer "Name of indirect buffer: " (current-buffer))) t))) (save-window-excursion (let ((newbuf (clone-indirect-buffer newname display-flag))) (switch-to-buffer-other-frame newbuf) ) ) ) )