I have been coming back every few months to look at the progress VisualEditor something that diapered long ago was the side-by-side view as seen at wmfblog:2011/12/13/help-test-the-first-visual-editor-developer-prototype I thought this was a very cool feature and hope they bring it back.
Topic on VisualEditor/Feedback/Flow
The side-by-side view we created in late 2011 was a technology demonstrator and not intended to be part of the long-term VisualEditor interface. In particular, the conversion between wikitext and HTML is non-trivial and to provide such a 'live' side-by-side would be very slow and unwieldily for users. Though it would help users learn wikitext quickly, this is not an objective of the VisualEditor (we're looking to replace it, not help users get accustomed and learn better).
Given that the Parsoid is built in nodejs, it's not absolutely impossible for side-by-side view to be brought back with the Parsoid running on the client side. However it would be so very slow and such a drag on the database servers that I do not expect us to ever build it, and if someone volunteered to develop it, I do not believe we would deploy it on Wikimedia's servers.
See also bugzilla:47779.