Once you've perfected Editorial as is, consider supporting two independent panes in the interface -- two views of the same file, or two different files. Maybe not so good for coding, but for text editing (which is what I do) it would be great. I imagine it could be done in Python, but not by me. Somebody else?
This cannot be accomplished with Python as the app is currently.
Although not a split screen I wanted something similar and wrote a workflow that shows a second file in a window with an option to insert part of the text into the existing text being edited. You also might find it helpful.
http://www.editorial-workflows.com/workflow/5796584834990080/qwf8Z0DBO7s
...dave
Basically, you would have to convert the markdown to html, and then view the html in a window view.
You can make the text in the view editable by using a "contenteditable" tag. I am thinking python can be used to convert the file back to markdown and save it in the current file.
Sounds very possible though.