I cannot find a way to change the setting, but I can tell you that some of the themes I have available are set to two columns, and some are not. Mocha, for instance, uses two columns and Butter Pecan does not. My wife is the one that normally uses B&N, so I don't know that those are standard themes, but judging from the names, I am going to assume they are.
So you might have to start with a different theme and re-customize the colors to your liking.
The Bandit