The blog index not just in Ethos 1 but in all of WordPress cannot be edited by Cornerstone or any page builder. The blog index displays your latest post items in chronological order. The items were dynamically generated by WordPress being displayed by the theme's template file for the index.
As you have mentioned that you already created 1/3 | 2/3 layout in your post, it still doesn't display in the homepage or your blog index because by default, the blog index, will only display excerpts of the post. If you want to display the full content, you will have to enable the "Full Post Content on Index" option in X > Theme Options > Blog > Content..
Hope this helps.