There isn't anyway to do what you are wanting here at wordpress.COM since what you want - two columns displaying posts from separate authors - would require access and modification of the underlying theme PHP script files, and we do not have access to those.
The closest you could get is to set up a couple categories with the names of the authors and then allow people to click on the category links to get a listing of the posts for each author.
With certain themes, you could even put links to the author's categories pages in the top navigation tabs if you wished: http://wpbtips.wordpress.com/2009/03/26/page-tabs-as-external-links/ .