I don't really know what the proper term is for this kind of feature so pardon the vague title.
All I can refer to as an example is with how in WriteMonkey, each page has it's "repository" page.
The way it works is that when you press alt+R, it switches to a "behind the scenes" page.
I don't know how it works but that's the type of feature I'm looking for except a multi-page one.
I know it's probably doable with multiple tabs and personal wikis and even a text comparing program however I feel this is the like a more convenient, more keyboard friendly way of comparing multiple drafts content per content. Paragraph per paragraph.
I also hope it's an application that doesn't require a huge learning step like Emacs or vim.
To narrow down the feature into specifics, here's how it would work:
1. You have a basic notepad (auto-save is nice but not required).
2. You then hit alt+r (or a keyboard shortcut of your own) and type your improved draft in there.
3. At this point it's no different from a notepad or WriteMonkey especially.
4. However, that second page has an extra special save button separate from the default save.
5. Click that and the first draft gets deleted and the second draft becomes the only draft but this time if you click the repository mode keyboard shortcut, it then now produces a repository mode for what was previously contents on the repository mode. (and the previous page is automatically deleted from the system)
6. The important part is that the two pages are instantly switchable via keyboard shortcut. Bonus points if the switch has an option to auto-set the cursor to the closest paragraph line of the previous page.
7. A visual description of this would be to imagine flipping between two pages of a notebook where the back page is where the edited content is. As the back page gets better, you rip out the page in front and then repeat the same method with the third page.