You can display two views of a single outline at the same time, in
different windows, by means of an alternative implementation of Outline
To do this, first load the library
noutline with M-x
load-library RET noutline RET. This loads the alternative
implementation of Outline mode. It provides the same command names and
key bindings as regular Outline mode, but it implements them
Then, to display a second view of an outline buffer, you must create an indirect buffer using M-x make-indirect-buffer. The first argument of this command is the existing outline buffer name, and its second argument is the name to use for the new indirect buffer. See section Indirect Buffers.
Once the indirect buffer exists, you can display it in a window in the normal fashion, with C-x 4 b or other Emacs commands. The Outline mode commands to show and hide parts of the text operate on each buffer independently; as a result, each buffer can have its own view. If you want more than two views on the same outline, create additional indirect buffers.
In a future Emacs version, the alternative
implementation will probably become the principal implementation.