Emacs has a couple of builtin ways of dealing with separate windows that are looking at the same file. The default method is to simply “open” the file again in a separate window. In this case you have two views into the same file. The windows may be looking at different parts of the file but any change made in one window appears in the other as well because both windows are using the same buffer.
Alternatively, you can open the second window by cloning the first buffer. That creates a second buffer with the same text and properties but they are distinct buffers that can have different point values, different narrowing, different major modes, different markers, different overlays, and different local variables. Nevertheless, any text or text property change made in one buffer is reflected in the other. Take a look at the Emacs manual for the details.
You’d think that those two options would cover whatever you needed to do but Sacha Chua has a post that describes a case not covered by either. In this case the user wants to display an SVG file in one window and the XML that generates it in another. The problem here is that the image is displayed with text properties so toggling the display in one window will cause it to affect the other window as well even if the other window is a cloned buffer.
Chua shows how to solve this by writing a bit of Elisp. The TL;DR is that she bypassed the code that checks if the target file is already opened in another buffer and simply creates a new buffer unconditionally. Of course, now you have to worry about syncing the text between the two buffers. Chua solves this by turning on global-auto-revert-mode so that when one file is saved, the other gets updated.
I’m always prattling about how having the Emacs source available from within the executable image coupled with the ability to make on the fly changes is one of Emacs’ magic powers. Chua’s solution is a nice example of this.