An initial attempt at "multifiles" as defined here.
Bind a key to
mf/mirror-region-lines-in-multifile, let's say
mark a part of the buffer and press it. A new *multifile* buffer pops
up. Mark some other part of another file, and press
C-! again. This
is added to the *multifile*.
You can now edit the *multifile* buffer, and watch the original files change. Or you can edit the original files and watch the *multifile* buffer change.
Saving in the *multifile* buffer saves all the original files.